Enlarger Lens F Stops - What are preferable values?

[ted_smith]

Having watched a video on expert-village about enlarging, the photographer briefly said "it's important to choose the right f-stop for corner to corner sharpness of your image" but did not ellabrorate. I have since been trying to work out what he meant.

On my 50mm Rodenstock Apo-Rodagon there are f-stops from f2.8 - f16. So how is one supposed to use the f-stops of an enlarger lens - I read it was not the same as on a camera? I read somewhere else that 'it should be used wide-open wherever possible' - I assume that means f2.8, but why?

Cheers

Ted

 

[Ian Grant]

The f stops are exactly the same as a camera lens. However enlarger lenses are usually best stopped down 2 or 3 stops, so your Rodagon f5.6 -f11.

Wide open it's unlikely to give optimum sharpness, and your exposure times would be far to short to control accurately, there are effects caused by the warm up time s you turn on an enlarger lamp.

Ian

 

[Trevor Crone]

Enlarger lenses are said to be optimised a stop or so from full aperture. However I've used my Rodenstock 105/f5.6 wide open at f5.6 and its as sharp as a tack corner to corner.

It is also said that if an enlarger lens is focused at full aperture when it's stopped down to make the exposure there is a slight shift in focus. Personally I've never noticed this with any of the lenses I own.

 

[srs5694]

I did some tests on several of my lenses, and found that most of my 50mm lenses performed best at about f/5.6 or f/8, which is about the middle of their ranges. The differences across the range varied from quite subtle (on my 6-element Nikon f/2.8 and 5-element Vega-11U) to easily noticed in the corners (on my 4-element lenses). This matches conventional wisdom -- namely, that the aperture makes very little difference on very good lenses, but that choosing the optimum aperture can be important on cheaper lenses. You mention that you've got an APO Rodagon, which is a very good lens, so you're likely to see very minor effects from your choice of aperture. (That is, effects of image sharpness; exposure time effects are of course another matter.)

 

[fschifano]

The key to your confusion is the lens, the APO Rodagon. I emphasize the APO part. These are designed to be used wide open, and deliver best performance at higher magnification ratios than their non-APO counterparts. To get more magnification, you naturally increase the head height. Increased head heights equate to lower illumination levels at the baseboard. Now throw in the fact that these lenses are also used to enlarged C-41 color negatives which are very dense, and you can see why they are optimized to work at or close to wide open. The common wisdom regarding enlarging lens apertures is valid for non-APO lenses. I don't know if there is any degradation to be realized by stopping an APO lens down a couple of stops. If there is, I doubt that it would be noticed. You can read the specs for the lens here: http://www.linos.com/pages/mediabase/original/rodenstock_enlarging_lenses_e_2281.pdf

 

[Pinholemaster]

What everyone said.

Focus at f/2.8.

Stop down to f/8 or f/11 or F.5.6 to expose your paper. The middle f/stop area is the sweet stop of the aperture scale for sharpness.

Then an image requires a lot of dodging, I tend to go to f/11. If it requires a lot of burning, I tend to go with f/5.6.

90% of the time I use f/8.0.

 

[ic-racer]

I don't know if there is any degradation to be realized by stopping an APO lens down a couple of stops. If there is, I doubt that it would be noticed. You can read the specs for the lens here: http://www.linos.com/pages/mediabase/original/rodenstock_enlarging_lenses_e_2281.pdf

I agree that the diffraction from stopping down a couple of stops may not be noticed in a print, but it is clearly seen with a grain magnifier.

Also, the link you gave to the Rodenstock 50mm APO contradicts what you posted. The MTF curves show that f5.6 is SUPERIOR to wide open. Also, the illumination falloff is much improved at f5.6. The Schneider 45mm APO behaves in a similar manner and I try to use mine at F8 based on the Schneider MTF curves. So I would not recommend using either of these APO lenses wide open unless you have to.

 

[Phillip P. Dimor]

In the community college darkroom, the instructor told us to stop the lens all the way down and then open it up two stops. This was presumably due to the enlargers never having been aligned, the easels having been dropped repeatedly and the enlarger lenses ranging from pretty good (el-nikkor 50mm f2.8 new style) to lousy (cheaper omega lens)..

 

[Ian Grant]

Like an earlier poster, I tested various lenses at different apertures. All performed best stopped down at least one, and most two stops, and remained excellent to f16, I didn't test past that except the LF lenses which were fine at f22. I tested Componons, Rodagons, El Rokkors, Durst Neonon (Pentax), Komuranons, all first rate lenses. However in the past I've used cheap lenses that were only sharp stopped down to f11 or f16.

Ian

 

[Martin Aislabie]

From the Rodenstock Web Site on the APO Rodagoan-N

"The optimum working apature is reached by stopping down by only 1 to 2 stops"

Which means in this case F4 or F5.6

Martin

 

[Gary Holliday]

- I read it was not the same as on a camera? I read somewhere else that 'it should be used wide-open wherever possible' - I assume that means f2.8, but why?

Cheers

Ted

It's amazing the amount of mis-information published on the web, but without reading the article I shouldn't attach any blame.

I tend to choose an aperture which suits the exposure of the B&W negative. I find it easier to work with an exposure of 10-20 secs - this gives me sufficient time to burn and dodge the print when necessary.

Don't get too bogged down on the technical data of lenses, it's not going to improve your printing skills. Focus and compose at the widest aperture and stop down two stops down/ f8 as suggested and you can't go wrong.

 

[Mick Fagan]

Having used the same enlarger lens in an industrial photo lab, where we had about 4 of them, along with their 90mm brother, I can say that you have one of the finest enlarging lenses ever made for 35mm photography, that money could buy, period!

You lens should work at almost any aperture and magnification so well, you will hardly notice the difference.

This lens, and the longer 90mm version, are optimised for extreme, accurate, optimal colour enlargements, B&W is a bonus.

Generally speaking you can stop down to aid your depth of focus (field) pretty much the same as you would stop down a camera lens, to allow either more or less image in critical focus, either side of the actual focus point.

In practice to make things easy, you could focus wide open, with no filtration in place, then stop down to your desired f/stop, add the filters, then expose at your pre-determined time.

I know that the sweet spot for this lens, magnification wise, is between 8 times and about 17 times enlargement of the negative. The hot spot of enlarging perfection of these lenses, is about 15 to 17 times magnification.

Think of your enlarging lens as a completely fixed, almost perfectly configured lens. The only way to focus is to move the lens backwards and forwards, meaning the glass elements stay at their factory set, optimal setting.

At one part of the enlarging magnification scale, the factory would have made the lens focus perfectly, where exactly, only they know, but generally speaking, and from personal experience, these lenses usually work best closer to their upper limits.

With this lens the factory may have made the actual perfect focus, somewhere around the 15 times enlargement setting. At this magnification, the lens will focus the three colour layers of the film onto the paper with pinpoint accuracy, you will (should) get extreme (virtually perfect) sharpness and colour fidelity. At other magnifications there is a difference, but realistically, one is nitpicking with this lens.

For B&W work, as long as your enlarger is aligned reasonably well, you should be able to realise the full potential of your camera system optics, as well as your own ability.

Mick.

 

[Anscojohn]

Does anyone still cut a piece of window screen, place it in the carrier, and make exposures on paper at various apertures to see when things sharpen up at the corners? The old lab rats that first introed me to darkroom back in the early sixties swore by (at) that technique. Of course, they also blew cigar smoke on a "light spot" to "bring it down a bit" so I cannot vouch for all their procedures.

 

[ic-racer]

Today I would beg, borrow, or steal the grain magnifier that shows the edges of the film (Peak No. 1), then you can check the grain at the edges of every negative you enlarge without resorting to 'experimental' printing (and you don't need the screen either).

 

[srs5694]

I'm not sure I trust what I see under grain focusers, at least not for comparing enlarger lenses, enlarger lens settings, etc. I say this because I've tried doing test prints from different lenses and at different f-stops (controlled to produce prints as near to identical in exposure as I could manage, of course). Sometimes a difference looks huge under the grain focuser but turns into nothing in the final print, and other times a difference that seems insignificant under the grain focuser becomes much more noticeable in the final print. Since most of us display our photos as prints rather than set up enlargers with grain focusers to display them, I trust the final print results. I can think of several reasons for these discrepancies, including different brightnesses (particularly when comparing f-stops), differences in the way the human visual system responds to the different types of images, etc.

 

[ic-racer]

I'm not sure I trust what I see under grain focusers, at least not for comparing enlarger lenses, enlarger lens settings, etc. I say this because I've tried doing test prints from different lenses and at different f-stops (controlled to produce prints as near to identical in exposure as I could manage, of course). Sometimes a difference looks huge under the grain focuser but turns into nothing in the final print, and other times a difference that seems insignificant under the grain focuser becomes much more noticeable in the final print. Since most of us display our photos as prints rather than set up enlargers with grain focusers to display them, I trust the final print results. I can think of several reasons for these discrepancies, including different brightnesses (particularly when comparing f-stops), differences in the way the human visual system responds to the different types of images, etc.

I share your experiences, and those are good observations. Perhaps some of these reasons are why aerial images are not frequently used to evaluate photographic optics. I got the impression he was asking about diffraction effects, which are easy to observe (though, as you point out, somewhat difficult to quantify without a print).

 

[ted_smith]

All

That all makes a lot of sense - thanks a lot. I think as I'm just getting into the world of printing I'll stick with the basic rule which seems to be "focus and compose at the widest aperture and stop down two stops down to f8" give or take. That is something that I was not doing before so hoorah.

One of the replies pleased me greatly - the one from Mick "Having used the same enlarger lens in an industrial photo lab, where we had about 4 of them, along with their 90mm brother, I can say that you have one of the finest enlarging lenses ever made for 35mm photography, that money could buy, period!" - you'd be amazed how much that has made my day. I stumbled across the lens by good fortune when RJ (a fellow APUG user and fine art photographer) very kindly offered to sell me his for a mere £50 ($100). He told me it was good and worth more like £150 but at the time I had no idea about enlarger lenses and I trusted what he said. So it does please me to read that it really is as good as he says. It makes a change from the usual scraps I pick up!

Thanks

Ted

 

[R W Penn]

Rodenstock apo 50mm and 80mm work best for me 3stops down. Very good lens.