What make RF lenses smaller than their SLR counterparts

[anta40]

So far, this is my conclusion: RF lenses are smaller than their SLR counterparts.

For example: Canon 50/1.8 LTM. Even smaller than the Nikkor 50/1.8 pancake, which is a nice slim lens.
Or Zeiss ZM 35/2, which is smaller than Nikkor 35/2 AI.

I wonder why. All of them are completely manual lenses, no electronic/motoric parts.
Perhaps RF lenses don't need many elements to project the light to film plane, because there's no mirror box on RF cameras?

 

[mshchem]

Good question. I have always assumed it was due in part from being farther away from the film?? Not sure if that's really the case. I know I like fast lenses on a slr for a brighter image.

 

[henryvk]

Afaik it's to do with how some lenses are made. Maybe someone with better knowledge can expain it in laypersons' terms?

In still photography, a single-lens reflex camera requires a space for the reflex mirror, imposing a limit on the use of wide-angle lenses of symmetric designs. The retrofocus lens addressed this situation by increasing the distance between the rear element and the focal plane, thus making wider-angle lenses usable while retaining normal viewing and focusing. Unless the reflex mirror were locked in the "up" position, blacking out the viewfinder, the rearmost element(s) of a non-retrofocus (symmetric wide-angle) lens would interfere with the movement of the mirror as it flipped up and down during exposure.[2]: 143 

Angénieux retrofocus - Wikipedia

en.wikipedia.org

 

[chriscrawfordphoto]

SLR lenses need the auto aperture stop down mechanism and the mechanisms or (for AF lenses) electronics that communicate with the camera's metering system.

 

[guangong]

SLR lenses need the auto aperture stop down mechanism and the mechanisms or (for AF lenses) electronics that communicate with the camera's metering system.

That just about says it all.

 

[halfaman]

So far, this is my conclusion: RF lenses are smaller than their SLR counterparts.

For example: Canon 50/1.8 LTM. Even smaller than the Nikkor 50/1.8 pancake, which is a nice slim lens.
Or Zeiss ZM 35/2, which is smaller than Nikkor 35/2 AI.

I wonder why. All of them are completely manual lenses, no electronic/motoric parts.
Perhaps RF lenses don't need many elements to project the light to film plane, because there's no mirror box on RF cameras?

There is relation between focal length, diameter and f-number. As the focal distance of a RF lens is shorter than in a SLR counterpart due to the lack of mirror, the diameter needed to achieve an aperture is smaller in a RF lens than in a SLR one. So RF lenses are shorter and smaller than SLR.

 

[Chan Tran]

But I see that Nikon Z lenses are larger than their F lenses.

 

[ph]

If you compare the LTM mount Voigtländer 90mm/3,5 Apolanthar with its Canon SLR mount version it is narrower and longer,While the mrginally faster 100/2,8Leitz Apomacro is much fatter and heavier due to its double helix allowing clos focus plus its autoaperture linkage. Olympus OM optics were the smallest Reflex camera optics if you do not count the Retina reflexDKLmount 80mm artar wth most aperture mechanics inside the camera . The fujita 135mm hold the size record in its class. Small aperture helps- Comparing like with like, the Summicrons for M and R illustrates differences in aperture mechanics. The sales promoting effect of "bigger is better" may have a role. Except in the case of extreme creations where more glass is needed.

p.

 

[halfaman]

But I see that Nikon Z lenses are larger than their F lenses.

But they are not the same lens, even they share the equivalent focal length and aperture.

Nikon AF-S 50 mm f/1.8 G has the typical double gauss with 6 elements in 4-5 groups. The Nikon Z 50 mm f/1.8 has a crowed design with 12 elements in 9 groups, displacing the original design far away the focal plane to accomodate the extra glass and adding what looks like a telephoto group to compensate the length increment.

 

[aw614]

But I see that Nikon Z lenses are larger than their F lenses.

Mirrorless seems to be a whole slew of difference on size, but its crazy how huge the lenses are on Full frame mirrorless, though some of the Chinese manual focus lenses aren't too bad in size.

 

[loccdor]

Break it down into a simpler example. A typical flange focal distance for a 35mm SLR is around 45mm. For rangefinders it's in the 35mm-28mm range.

A very simple lens design should have a focal length equal to its actual physical length. Pinholes do.

To make a wide angle lens that is longer than its focal length, as is required by the larger flange focal distance of SLRs, or a telephoto lens that is shorter than its focal length, takes a more complicated design with more lens elements.

The details already mentioned about the lens metering/stopping down communication mechanism required on SLRs but not on RFs is also true.

 

[reddesert]

There is relation between focal length, diameter and f-number. As the focal distance of a RF lens is shorter than in a SLR counterpart due to the lack of mirror, the diameter needed to achieve an aperture is smaller in a RF lens than in a SLR one. So RF lenses are shorter and smaller than SLR.

I do not really agree. A (for example) 50mm f/2 lens requires an aperture that is 50/2 = 25mm diameter as viewed from the front of the lens, whether the lens is a small-bodied rangefinder lens or large bodied SLR lens. You can check this yourself by laying a ruler across the front of the lenses and looking at the apparent aperture diameters.

The reasons have been mentioned above:
1. 35mm SLR lenses that are wideangle need to be retrofocus to clear the mirror, and even normal lenses such as 50-55mm need to be slightly retrofocus. The retrofocus (reverse telephoto) design results in larger diameter elements especially in the front. Although the apparent diameter of the aperture is the same between retrofocus and non-retrofocus, and the physical diameter is pretty similar.
2. SLR lenses have more mechanics in them: auto aperture mechanism, meter coupling links, and so on.

 

[Oren Grad]

For a nice example, compare the SMC Pentax-FA Limited 43mm f/1.9 with the SMC Pentax-L Special 43mm f/1.9. Same glass, one version is in SLR K-mount with auto diaphragm and linkages for AF drive, the other in Leica screw mount with manual diaphragm and manual focus helical only. The former is short and fat, the latter long and slim; the length of the latter reflects the fact that the lens was designed for the K-mount flange-to-film distance so that the L version needs extra empty tube length to position the glass properly.

 

[snusmumriken]

For a nice example, compare the SMC Pentax-FA Limited 43mm f/1.9 with the SMC Pentax-L Special 43mm f/1.9. Same glass, one version is in SLR K-mount with auto diaphragm and linkages for AF drive, the other in Leica screw mount with manual diaphragm and manual focus helical only. The former is short and fat, the latter long and slim; the length of the latter reflects the fact that the lens was designed for the K-mount flange-to-film distance so that the L version needs extra empty tube length to position the glass properly.

And yet even those lenses are huge compared with the very respectable 40mm f/2 lens of the Rollei 35S, covering the same format full-frame.

Some of the explanation for lens size must have to do with whatever mount size the manufacturer has committed to. When Olympus released the OM-1, its first camera, the lens mount was designed in proportion to the innovatively small body, and its lenses were dinky. Nikon responded with the FE, but to retain compatibility with existing Nikon bodies, they stuck with the larger lens mount, and no dinky lenses.

 

[Oren Grad]

Some of the explanation for lens size must have to do with whatever mount size the manufacturer has committed to. When Olympus released the OM-1, its first camera, the lens mount was designed in proportion to the innovatively small body, and its lenses were dinky. Nikon responded with the FE, but to retain compatibility with existing Nikon bodies, they stuck with the larger lens mount, and no dinky lenses.

Actually, the diameter of the OM mount is larger than that of the F mount, not smaller. The Pentax K mount is the same diameter as the Nikon F mount, but the Pentax M-series lenses are very compact, like the OM lenses. These are all design choices made for other technical and marketing reasons - the mount size was not a binding constraint on the overall size of the lens.

 

[reddesert]

And yet even those lenses are huge compared with the very respectable 40mm f/2 lens of the Rollei 35S, covering the same format full-frame.

Some of the explanation for lens size must have to do with whatever mount size the manufacturer has committed to. When Olympus released the OM-1, its first camera, the lens mount was designed in proportion to the innovatively small body, and its lenses were dinky. Nikon responded with the FE, but to retain compatibility with existing Nikon bodies, they stuck with the larger lens mount, and no dinky lenses.

The optics in the 43/1.9 lenses Oren mentioned are designed for an SLR, so they have a slightly retrofocus design (especially for a lens as short as 43mm), which makes the optics bigger than the 40/2 in the Rollei. Oren's point was that you can see how the SLR mechanics cause the two Pentax lenses to be different sizes even though the optics are the same.

Take an OM mount SLR and a Nikon F mount SLR and line them up to see which is bigger. If you don't have one, I did it myself. The OM lens mount has a slightly larger throat. (If one cares, the OM mount flange is ID/OD of about 47/59mm and the Nikon F is about 46/55mm.) The OM was designed to be compact, and they made the 50/1.8 smaller compared to typical normal lenses like the Nikon 50/2 or early 50/1.8. However, Nikon designed the later 50/1.8 series E to be more compact than even the OM 50/1.8; other companies also came up with compact designs.

When you look at a not-the-fastest SLR normal or moderate wide lens, like a 50/1.8, 50/2, 35/2.8, etc, the glass is significantly smaller than the lens barrel. Some of that is probably for SLR mechanics and some for ease of handling. An extreme example is the typical slowish macro lens like a 55/3.5. The glass is much smaller and shorter than the lens barrel, and the glass is often set deep into a barrel that is almost a lens hood. I suspect one reason is that the long focus helical travel needed for such a macro lens makes it physically long even in front of the glass.

 

[Oren Grad]

The optics in the 43/1.9 lenses Oren mentioned are designed for an SLR, so they have a slightly retrofocus design (especially for a lens as short as 43mm), which makes the optics bigger than the 40/2 in the Rollei. Oren's point was that you can see how the SLR mechanics cause the two Pentax lenses to be different sizes even though the optics are the same.

Precisely - thank you.

+1 to all of your other points.

 

[Jonathan-sv]

There is a lot of variation even in RF lenses due to lens design. These are all 35 mm lenses I have used on a Leica M. The Voigtländer 1.7 on the left is so long I stopped using it because it blocks part of the finder. I started with the Jupiter 12 in the middle, but now use the Summicron on the right.

 

[koraks]

There is relation between focal length, diameter and f-number. As the focal distance of a RF lens is shorter than in a SLR counterpart due to the lack of mirror, the diameter needed to achieve an aperture is smaller in a RF lens than in a SLR one.

I think you're confusing focal length and physical length here. The aperture on e.g. a f/2.0 on a 50mm lens will always be 25mm as projected through the front element.

The main difference in size will be with wide angle lenses, where an SLR requires a retrofocus design while a RF doesn't.

Some construction aspects stemming from functional requirements (esp. aperture coupling and control) as mentioned by @chriscrawfordphoto will also play a role. The electronics in a modern SLR lens are quite compact and don't add all that much bulk (and certainly not much weight).

 

[chriscrawfordphoto]

I think you're confusing focal length and physical length here. The aperture on e.g. a f/2.0 on a 50mm lens will always be 25mm as projected through the front element.

The main difference in size will be with wide angle lenses, where an SLR requires a retrofocus design while a RF doesn't.

Some construction aspects stemming from functional requirements (esp. aperture coupling and control) as mentioned by @chriscrawfordphoto will also play a role. The electronics in a modern SLR lens are quite compact and don't add all that much bulk (and certainly not much weight).

I forgot to mention in my earlier posts the autofocus mechanism in modern lenses. All modern AF lenses have built in motors to drive them, and older lenses by Nikon, Pentax, and Minolta had geartrains driven by a motor in the camera body to focus the lens. That stuff all adds bulk too.

 

[Mark J]

Halfaman got it pretty much correct a few posts ago. It is the mirror space ( ~ 38mm ) on SLR lenses which make them bigger for the same focal length and f/number.
This is somewhat confused by many RF lenses being faster or aiming for a higher MTF spec, but for an equivalent lens, it would hold.
However, this advantage is mainly for primes and zooms that are of standard or wide angle ( at the short end of zoom ) , not so much telephotos.

( ps. I'm a lens designer since 1987... )

 

[Mark J]

The thing about the mirror space is that it creates asymmetry in the design.
Asymmetry in the stop position and in the power balance from front to back.

With primes, the problems already start at 50mm focal length.
A 50mm f/2, or 50 f/1.8 is a fairly small unit and the 38mm back focus is a reasonable fraction of the 50mm, so an SLR lens of this spec is almost the same as an RF design, and they both achieve excellent performance both at full aperture and in the field when stopped down.

However at 50mm f/1.4, this is an inherently more bulky lens because of the aperture and the number of elements ( 7 ) required. Even using very high-index glass, as they do, the natural back focus of a lens like this designed without constraint would be 35mm or less. So, you have to 'force' the design a little to achieve the mirror space.
So instead of the front group and the rear group having roughly the same power, the front group now has to be a little longer in FL and the rear group shorter.
The stop moves back a few mm within the lens form - which enlarges the front element slightly.
The asymmetry in the powers makes it harder to correct distortion to zero, and lateral colour in the field.

The trade-offs are not too severe though, both and RF 50/1.4 and and SLR 50/1.4 can be very good, but close inspection of the two designs and their performance measures, will show that the SLR lens typically has 1.5 to 2% barrel distortion whereas the RF lens will be close to zero.
The MTF in the field, particularly when stopped down, will be slightly inferior towards the corner, in the SLR lens.
This can all be confirmed by looking at the MTF and distortion of a Leica R 50/1.4 and a Summilux-M 50/1.4

For very wide angles, the design differences can be very obvious.
A 19mm lens has to have a back-focus distance of twice its focal length in order to work with an SLR.
This can only be achieved by using huge amounts of negative power up-front, with a much more positive group in the rear.
Given the strongly diverging field angles of the corner bundles, this makes the front group pretty large.

 

[loccdor]

Very detailed and informative, thank you!

 

[MattKing]

One further likely factor - the overall handling characteristics of lens and camera combinations.
Every lens design involves a plethora of design compromises.
If one is designing a lens to fit a line of SLRs, the size of the camera bodies make very small lenses lenses less advantageous. It can actually be more difficult to handle combinations of tiny lenses on larger camera bodies.
In comparison, relatively small rangefinder camera bodies call for smaller lenses - together they handle well.
If, for example, Leica had started out with the M5, the lens line may have started out larger.

 

[Mark J]

Well, in the case of the M cameras, the smaller lenses are usually achieved just by them being a slower f/number.

One other beneficial factor that can make some of the mirrorless lenses more compact overall is that the bayonet sizes have been opened out quite a bit. This gives the lens designs more chance to be unhindered in their form.