Exposure compensation for small apertures

[JohnF]

I am working my way through Leslie Stroebel’s book View Camera Technique.
On page 80, he supplies a table of adjustments to exposure at small aperture for shorter shutter speeds. For example, a lens with a maximum aperture of f5.6 stopped down to f16 with a shutter speed set at 1/500 second, should be given an extra 1 stop of exposure. Any shutter speed at 1/125 or above needs some such adjustment.This is quite a significant additional exposure and one that I was not aware of.
I use FP4+ and will generally never use shutter speeds that come close to 1/500 of a second, but it is an intriguing piece of information.
Anyone else ever been in a situation that they have had to make such an adjustment?

[Vaughn]

Never read the book, never heard of such corrections. Not the typical resiprocity failure. If anything those short shutter speeds are more likely to be off...and if anything slightly longer than dial speed.

Looking forward to other's input on this.

[ic-racer]

The correction is real and well known, but there isn’t film fast enough for the situation to arise in most large format photography.
Red= Smallest aperture (eg. f64 @ 1/500)
Blue = widest aperture (eg. f5.6 @ 1/500)



I’d also emphasize that the above only holds for 1/500 (or the fastest speed.)
For example a 2 second exposure looks like the Red trace at any aperture

[Michael R]

It’s basically because of lens shutter inefficiency (ie the fact a shutter/aperture is obviously not instantly fully open or closed but needs to move through those transitions). For a given shutter speed, the larger the aperture that needs to be opened for the exposure, the lower the shutter efficiency, and vice versa. Lens shutters are apparently calibrated assuming maximum aperture, but since small apertures result in higher efficiency, you get more exposure at small apertures than indicated.

However this is all negligible unless you are using short exposure times, say 1/250.

[Ulophot]

I am working my way through Leslie Stroebel’s book View Camera Technique.
On page 80, he supplies a table of adjustments to exposure at small aperture for shorter shutter speeds. For example, a lens with a maximum aperture of f5.6 stopped down to f16 with a shutter speed set at 1/500 second, should be given an extra 1 stop of exposure. Any shutter speed at 1/125 or above needs some such adjustment.This is quite a significant additional exposure and one that I was not aware of.
I use FP4+ and will generally never use shutter speeds that come close to 1/500 of a second, but it is an intriguing piece of information.
Anyone else ever been in a situation that they have had to make such an adjustment?

I just looked this up. Unless I am reading wrong, the film receives extra exposure when the lens is stopped down, because the shutter blades have, upon partial opening, already uncovered the totality of the area of the smaller aperture opening. In my (3rd) edition, pg. 89-90, Stroebel says that this can result in as much as a stop more. His suggests a test comparing 5.6 with an ND filter with 16 or 22 at the same fast shutter speed.

[Doremus Scudder]

This gets a bit complicated. I think JohnF may have it backward... FWIW, my copy of Stroebel's View Camera Technique doesn't have the table of corrections JohnF refers to, on p. 80 or anywhere else I can find. I have the 4th edition.

The efficiency of the clockwork shutters at different apertures seems to be what is at issue here (note: this doesn't apply to focal-plane shutters). A quick explanation:

A between-the-lens clockwork shutter uncovers the aperture from starting at the center and then progressively outward till the shutter is wide open. The shutter, after staying open for the needed amount of time (if any), then closes from the outside in.

One can see that a small aperture will be completely uncovered before the shutter reaches its fully open position. When the aperture is set wide open, it will remain partially covered until the shutter is fully open.

Therefore, for any given shutter speed, the small aperture is transmitting 100% of the light it can pass for a large portion of the time the shutter is open. With a large aperture, the percentage of transmittance is 100% only for the brief time when the shutter is fully open; it is partially covered for all the time the shutter moving between the closed and open positions (and back).

If you had a scenario in which you could make the same exposure with the same shutter speed and two different apertures (say we compensate for exposure with a neutral-density filter), then the exposure made at the smaller aperture would get more total exposure than the one made at the larger aperture because the aperture would be transmitting 100% of its potential for a longer period. This, of course, assuming that the shutter speeds are accurate.

According to Stroebel, "Shutters are normally calibrated so that the marked exposure time is equal to the effective exposure time at the maximum diaphragm opening." [Stroebel, View Camera Technique, 4th ed. p.89.]. What this is saying is that he assumes a "standard" in which shutter timing is actually slower than marked to compensate for the effect described above in order to bring actual shutter timing into agreement with the effective transmission of the lens.

If this is true (and this is a big "if" these days), then one would have to reduce exposure when using small apertures (not increase exposure as JohnF reports). The amount of the reduction would depend on the shutter speed, of course. For longer speeds, during which the shutter stays in the open position for most of the exposure, the amount of correction would be a smaller percentage of the total exposure than for speeds during which the shutter simply moves quickly to its open position and then begins to close immediately. This latter only happens starting at 1/30th or 1/60th second and faster for most shutters.

This means the real concern would be compensating for faster shutter speeds with smaller apertures. And, since most clockwork shutters are inherently inaccurate at faster shutter speeds (even on well-adjusted shutters in good working condition speeds of 1/500th and faster can easily be a stop slower than marked), this error would be compounded.

All this is somewhat moot if we're using black-and-white negative materials that tolerate overexposure well. With color transparency films, it could be an issue. But, if we calibrate our shutter speeds using real time optical methods using typical taking apertures (i.e., not wide open) and use actual speed instead of what is marked, much of the error can be eliminated. And, if we spend a bit of time keeping good notes and are aware of the variations in shutter efficiency at different apertures, we can compensate for variances by tweaking exposure as needed.

In my case, I don't vary much from the f/22-32 mark most of the time for my taking apertures, so whatever error I have that is aperture-related is fairly constant and I've dealt with it long ago by arriving at an effective film speed. Plus, I test my shutters at f/22 and then use an average of the actual shutter speeds, rounded to the nearest 1/3-stop, instead of marked speeds to do my best to avoid discrepancies from the marked vs. actual shutter speeds.

@JohnF: I'd love to see the table that you have in your Stroebel copy. Could you post it here?

Best,

[Drew Wiley]

Most high-speed work is done by flash. And few LF leaf shutters are anywhere near accurate at their highest speed of 1/500 anyway; I've only ever had one which was. I can't ever recall taking a view camera exposure faster than 1/60th, at least that I found satisfactory. So most of this is not anything to worry about in typical LF photography. If one is into scientific photography instead, of course, you'd want to be aware of all the potential variables.

[BrianShaw]

Shutter efficiency is an important consideration when using flash bulbs. I’ve never in my life heard it’s a significant consideration for normal daylight photography no matter what aperture is being used.

[Michael R]

Correct, the table indicates the amount to reduce exposure (vs indicated) for smaller apertures where efficiency is improved relative to the calibration assumption.

In other words, the shutter calibration includes some extra exposure to compensate for shutter inefficiency, and this can result in overexposure at small apertures where efficiency is high.

According to the table this is just noise until you get to shutter speeds of 1/60 and even then you’re looking at 1/4 stop, which is inconsequential. You only get to 1/2 stop in the 1/250 range.

It isn’t anything to worry about under normal conditions.

This gets a bit complicated. I think JohnF may have it backward... FWIW, my copy of Stroebel's View Camera Technique doesn't have the table of corrections JohnF refers to, on p. 80 or anywhere else I can find. I have the 4th edition.

The efficiency of the clockwork shutters at different apertures seems to be what is at issue here (note: this doesn't apply to focal-plane shutters). A quick explanation:

A between-the-lens clockwork shutter uncovers the aperture from starting at the center and then progressively outward till the shutter is wide open. The shutter, after staying open for the needed amount of time (if any), then closes from the outside in.

One can see that a small aperture will be completely uncovered before the shutter reaches its fully open position. When the aperture is set wide open, it will remain partially covered until the shutter is fully open.

Therefore, for any given shutter speed, the small aperture is transmitting 100% of the light it can pass for a large portion of the time the shutter is open. With a large aperture, the percentage of transmittance is 100% only for the brief time when the shutter is fully open; it is partially covered for all the time the shutter moving between the closed and open positions (and back).

If you had a scenario in which you could make the same exposure with the same shutter speed and two different apertures (say we compensate for exposure with a neutral-density filter), then the exposure made at the smaller aperture would get more total exposure than the one made at the larger aperture because the aperture would be transmitting 100% of its potential for a longer period. This, of course, assuming that the shutter speeds are accurate.

According to Stroebel, "Shutters are normally calibrated so that the marked exposure time is equal to the effective exposure time at the maximum diaphragm opening." [Stroebel, View Camera Technique, 4th ed. p.89.]. What this is saying is that he assumes a "standard" in which shutter timing is actually slower than marked to compensate for the effect described above in order to bring actual shutter timing into agreement with the effective transmission of the lens.

If this is true (and this is a big "if" these days), then one would have to reduce exposure when using small apertures (not increase exposure as JohnF reports). The amount of the reduction would depend on the shutter speed, of course. For longer speeds, during which the shutter stays in the open position for most of the exposure, the amount of correction would be a smaller percentage of the total exposure than for speeds during which the shutter simply moves quickly to its open position and then begins to close immediately. This latter only happens starting at 1/30th or 1/60th second and faster for most shutters.

This means the real concern would be compensating for faster shutter speeds with smaller apertures. And, since most clockwork shutters are inherently inaccurate at faster shutter speeds (even on well-adjusted shutters in good working condition speeds of 1/500th and faster can easily be a stop slower than marked), this error would be compounded.

All this is somewhat moot if we're using black-and-white negative materials that tolerate overexposure well. With color transparency films, it could be an issue. But, if we calibrate our shutter speeds using real time optical methods using typical taking apertures (i.e., not wide open) and use actual speed instead of what is marked, much of the error can be eliminated. And, if we spend a bit of time keeping good notes and are aware of the variations in shutter efficiency at different apertures, we can compensate for variances by tweaking exposure as needed.

In my case, I don't vary much from the f/22-32 mark most of the time for my taking apertures, so whatever error I have that is aperture-related is fairly constant and I've dealt with it long ago by arriving at an effective film speed. Plus, I test my shutters at f/22 and then use an average of the actual shutter speeds, rounded to the nearest 1/3-stop, instead of marked speeds to do my best to avoid discrepancies from the marked vs. actual shutter speeds.

@JohnF: I'd love to see the table that you have in your Stroebel copy. Could you post it here?

Best,

[maltfalc]

adding this to my list or reasons to like my speed graphic.