f/stop question?

stradibarrius · Apr 6, 2010

I understand the function of the selection of aperture while shooting but other than controlling the amount light, what else does different apertures do when printing?

Mats_A · Apr 6, 2010

As I understand it, cheaper lenses are not so good "in the corners" so stepping them down creates a shareper image.

r

mats

ic-racer · Apr 6, 2010

The aperture ring on an enlargeing lens allows you to open up for framing, focusing and composing. If they just made the lens for the optimum aperture, it would be dim and difficult to focus.

If you don't have the MTF curves for your lenses, the recommendation to 'close down 2 or 3 stops' from wide open is good advice.

tkamiya · Apr 6, 2010

If you have a lens (of any kind) that is wide open, you have very narrow depth of field. On enlarger, that means:

1) slight mis-focus caused by operator error
2) paper that is not completely flat
3) enlarger that is not correctly aligned
4) lens that does not produce flat field of focus (ie. lens that is not corrected to center/edge focus)

can have an effect on your result. By stopping down (to a smaller aparture), you can mask these issues.

I did some experiment. Based on experience, the distance between film and lens has much larger impact on focus than distance between lens and the baseboard. In fact, you could vary the latter by as much as 1cm and see no visible difference under magnification. Also, if you open up too much, the exposure time will get so short, precisely controlling exposure by time will be difficult. If you have 8 seconds at f/8, by going f/5.6, you have 4 seconds. At f/4 you have 2 seconds, etc.

markbarendt · Apr 6, 2010

Controlling the volume of light is the biggie for most.

It does increase Depth of Focus too.

This is important if there is any enlarger misalignment. "Misalignment" is not always a "problem" or "defect" either; it can be intentional.

Try this experiment, set up a negative in your enlarger that has a horizontal subject. (Any angle subject works in real life once you get the concept.)

Put an 8x10 easel in place and prop up the top edge of the easel with a 35mm film canister or something.

With the lens at it's largest aperture focus the subject as best you can. (A grain focuser won't help here.)

Now stop down a bit at a time, more and more of the photo will come into focus.

You can use this technique to, in a controlled manner, blur away trash and unimportant stuff.

This easel tilt can also help you "square up" the parallax of any element you choose.

Some enlargers were made from or like view cameras where the lens can swing and tilt to allow even larger corrections.

williamtheis · Apr 6, 2010

beware of diffraction limits! as the aperature goes down below f/16 or so, the lens actually gets less sharp!

keithwms · Apr 6, 2010

Falloff is the major issue, if you are pushing the lens coverage to its limits. You want smooth illlumination right to the corners and for large prints you may need to stop down substantially.

Regarding sharpness, I find that to be less of an issue than I thought it would be. Use a grain focuser and with many films you will see the grain very sharply even when the enlarging lens is stopped down to f/22 or such. I worry about this a lot more when I am enlarging negs e.g. to ortho film.... but I find that paper (esp. the textured fiber papers that I prefer) won't reveal much if any difference between prints shot at f/11 and f/22.

Indeed most enlarging lenses are not terribly sharp in the corners, when shot wide open. I rarely find it noticeable in the final print, though. Probably better to pay attention to how sharp your shooting lens is in the corners!

RalphLambrecht · Apr 6, 2010

stradibarrius said:
I understand the function of the selection of aperture while shooting but other than controlling the amount light, what else does different apertures do when printing?

You received many good answers already. Here are my two cents:

The typical rule for lenses is that the open aperture is only good for framing and composition. One must stop down a couple of stops to get the best lens performance. While stopping down lens aberrations are reduced but aperture diffraction is increased. A 35mm lens offers the best compromise between the two somewhere between f/8 and f/11, regardless of the widest aperture the lens may offer, see attached graph. For many enlarging lenses, this means to stop down no more than one stop. For many taking lenses, it means 3 stops or more.

The aperture also controls the depth of field at the baseboard and the depth of focus at the negative plane. As stated already, the depth of field is relatively large and of little concern (forget putting a piece of paper under the focus finder, it's meaningless). However, the depth of focus is very small and critical, that why glassless carriers are very difficult to make a truly sharp image with.

ic-racer · Apr 6, 2010

RalphLambrecht said:
Nice graph!

Steve Smith · Apr 6, 2010

RalphLambrecht said:
The aperture also controls the depth of field at the baseboard and the depth of focus at the negative plane.

As an enlarger is really just a macro camera with the negative as the subject and the paper taking the place of the film, wouldn't it be more correct to say depth of focus at the baseboard and depth of field at the film.

p.s. I must point out that I'm not arguing that this is the case but just asking the question.

Steve.

RalphLambrecht · Apr 6, 2010

Steve Smith said:
As an enlarger is really just a macro camera with the negative as the subject and the paper taking the place of the film, wouldn't it be more correct to say depth of focus at the baseboard and depth of field at the film...

Steve

It's just a matter of definition, and one could call it either way, but it does make some sense to consistently refer to the tolerance band around the film or negative as the 'depth of focus', and call the other the 'depth of field'.

Q.G. · Apr 6, 2010

RalphLambrecht said:
It's just a matter of definition, and one could call it either way, but it does make some sense to consistently refer to the tolerance band around the film or negative as the 'depth of focus', and call the other the 'depth of field'.

You could also consistently refer to the band of tolerance around the subject (in "object/subject space" : in this case the negative in the stage) as the "depth of field", and that around the recording medium (in "image space" : in this case the paper under the enlarger) as "depth of focus".
Just a matter of choice indeed.

Steve Smith · Apr 6, 2010

That's the point I was making. I always think of the negative in an enlarger as the subject and the paper as the film when thinking in camera terms.

However, it's just a choice of words really and it is understandable either way.

Steve.

Q.G. · Apr 6, 2010

On a tangent: it helps to think about it that way round too when considering using enlarging lenses in photomacrography.
These lenses are made to work best with the shorter conjugate distance towards the subject (the negative), the longer towards the recording medium (the paper). When putting them on a bellows facing the 'right way round' they are in fact the wrong way round. The advice to 'reverse mount' a lens also holds for enlarging lenses.
Though it is very rare to hear that: what people are told regularly is to 'reverse mount' a normal camera lens, or use an enlarging lens.

sandermarijn · Apr 6, 2010

I've just been staring at that graph for a few minutes (longer?). Very nice graph indeed, but not intuitively clear to me. Those grey lines, are they taken from the typical 6-element lens MTF data? "Actual" in that graph means "as computed from typical enlarger lens specs"?

The coloured bars seem to indicate that in the 35mm format only some kind of (purely hypothetical!) diffraction-limited superduper lens @ full aperture would be able to produce results (resolution) that stand the most of scrutiny. That's pretty shocking, I never knew that.

Can you elaborate a bit on that "actual" part?

RalphLambrecht · Apr 6, 2010

sandermarijn said:
I've just been staring at that graph for a few minutes (longer?). Very nice graph indeed, but not intuitively clear to me. Those grey lines, are they taken from the typical 6-element lens MTF data? "Actual" in that graph means "as computed from typical enlarger lens specs"?...

Sorry about the confusion, the graph is about taking lenses, not enlarging lenses, and yes, the data is from typical lenses (Nikon, Hasselblad, Schneider) on typical film after normal processing.

sandermarijn said:
...The coloured bars seem to indicate that in the 35mm format only some kind of (purely hypothetical!) diffraction-limited superduper lens @ full aperture would be able to produce results (resolution) that stand the most of scrutiny. That's pretty shocking, I never knew that...

True indeed, that's why we see a huge difference between 35mm and 6x6 enlargements but a smaller difference between 6x6 and 4x5.

sandermarijn · Apr 7, 2010

RalphLambrecht said:
Sorry about the confusion, the graph is about taking lenses, not enlarging lenses...

That's only good because now I have an excuse to reexamine that wonderful graph

Now that I do: the graph also shows that resolution wise 6x6 is sort of optimum (if no cropping, assuming good lenses, etc). Going smaller you loose resolution, bigger is overkill. Of course that's only resolution, tonality would be yet another story.

It suprises me also how close in optimum resolution the MF lens is to the 35mm lens. I always thought that the difference was bigger. Comparing my Contax G 45mm to the Hasselblad C T* 80mm (both Zeiss), the difference feels like more (but then there is so much involved in these subjective comparisons that such an 'intuitive mistake' should not be surprising I guess).

RalphLambrecht · Apr 7, 2010

sandermarijn said:
...It suprises me also how close in optimum resolution the MF lens is to the 35mm lens. I always thought that the difference was bigger. Comparing my Contax G 45mm to the Hasselblad C T* 80mm (both Zeiss), the difference feels like mor...

For our comparison, we used an average of the best lenses we could get our hands on. For 35mm, we used Nikon and Leica lenses. For 6x6, we used Mamiya and Hasselblad, for 4x5 Nikon, Schneider and Rodenstock.

Are you comparing your lenses at the same magnification? If you compare 8x10 prints made from both, the difference will be bigger, because the 35mm neg is enlarged more, but comparing their negs under a microscope, the difference will be smaller.

Also important to note is that this is not a lens test. It is a system test, the system being lens, film and development. Aerial images of lenses have a much higher resolution, but we aimed a more practical comparison.

sandermarijn · Apr 7, 2010

RalphLambrecht said:
Are you comparing your lenses at the same magnification? Also important to note is that this is not a lens test. It is a system test, the system being lens, film and development. Aerial images of lenses have a much higher resolution, but we aimed a more practical comparison.

I inspect all my negs under a loupe. Maybe this is a bit silly but I like doing it, and it complements the scan that I always make before (in case of some negs) making a print.

The loupe inspection always tells me that the 35mm-system lens has higher resolving power/better micro contrast/what-you-want-to-call-it than the MF lens. (This is comparing Contax G lenses to Hasselblad V). Which is obviously a judgement unfair towards the MF lens, because images taken with that lens will see less enlargement than the 35mm negs.

The grey lines in your graph are pure resolution figures of each system, i.e. not taking into account that the smaller system will face tougher scrutiny through enlargement. This is the same case as inspecting with a loupe- no normalization for final print size there either. I was surprised with your graph because, unlike my loupe-comparison, those plots show not a huge difference in resolving power between systems, at least not as much as I think I see with my loupe.

The problem is of course that my loupe inspections are subjective and unquantified, whereas your graphs are the opposite. I therefore have no right to be surprised, yet I still am

I am aware of yours being a system test, not (just) a lens test. That's the thing that makes your graph so interesting. Normally people say "oh, that Hasselblad lens is no good, just look at the MTF- even my Canon beats that by a mile" and they don't mention the difference in magnification. Your graph shows MTF *and* takes into account magnification (through the bars).

RalphLambrecht · Apr 7, 2010

I understand now. Thanks for the clarification.

f/stop question?

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