Physics 101 - Optics

[venchka]

This week I have read more than one post about stopping a lens on a DSLR down to f/22. Jeff wants to know if slow shutter speeds make for sharper, more detailed photos. At least one veteran member once said something to the effect that back in the medium format day, "stop down to f/16". Are any or all of these ideas right? Wrong? Who knows? Who cares?

Well, it depends. Some basic principles...

1. As sensors shrink, so does the depth of field.

2. As apertures close, diffraction gremlins start degrading the image.

3. Most lenses perform best at apertures in the middle of their range of apertures.

4. Shutter speed and aperture are inversely related.

Jeff's observation of slower shutter speeds yielding more detail is correct, but it's the smaller aperture that actually yields the sharper image. Up to a point. I haven't seen every lens test ever published, but I've seen enough to know that ALL lenses start to loose resolution at f/16. Even Bazillion $ German lenses for all formats do it. The diffraction Gremlins may not show up in an 8x10. You can bet they start showing up in 11x14s or 12x18s from a small digital sensor. On the other hand, if you're shooting 8x10 film and making contact prints, feel free to use the f/64 stop on your Rodenstock Super Gizmo lens.

Here's an example of one of the great lenses of all time. The Zeiss 80mm/2.8 Planar found on myriad Hasselblad, Rollei, etc. medium format cameras. The numbers are for center, middle and edge resolution.

80mm f/2.8 Zeiss Planar

68 68 42 f/2.8
76 96 68 f/4
68 76 68 f/5.6
96 96 76 f/8
96 85 68 f/11
68 76 60 f/16
54 60 48 f/22

Notice the drop off at f/16 and f/22. DOF of field issues aside, the lens performs better at f/5.6 than at f/16. I can assure you that DSLR lenses and sensors perform in similar manner and probably a lot worse beyond f/11. Large format lenses show similar results. However, the Gremlins aren't as obvious due to the lower magnification required for printing.

Here's the link to the whole report:

Medium Format Lens Test

OK, so much for using very small apertures.

Back to Jeff and his slow shutter making better pictures. As was pointed out in the original thread, slow shutters mean small apertures. This gives you more depth of field (already shallow because of the small sensor) and increased resolution. Up to a point. Somewhere between f/11 and f/16 you reach a point of diminishing returns. DOF increases but diffraction around the aperture blades robs you of real resolution.

What's the real deal? Jeff was partially right. The f/22 folks went too far.

The optimum for detail, DOF and overall image quality (overlooking the creative benefits of shallow DOF), is minimum ISO setting, moderate f-stop, corresponding shutter speed, rock solid support, mirror lock up and all the sensor acerage you can get your hands on.

Thus concludes the boring lesson for today.

Cheers!

[kenw]

well, i found it helpful. feel free to be so boring more often!

thanx.

[LanceHaynie]

Thanks for this!! I often forget that the sensor is not full frame, and the whole where you get optimum detail parts.

Though its not a boring lesson, I often find I like the science behind photography just as thrilling as putting it into action, That may only be me.

[ShutteredEye]

Like my wife tells me...it's all about the sweet spot...

[venchka]

Robert, Grinning.

Thanks!

The science can be interesting. Don't let the nuts & bolts get in the way of the doing and the making of photographs.

[KJ Smith]

Wayne,

Check #1

Kevin

[Dobick]

I understand most of the things that you posted, and knew them from a previous lesson learned on this forum, but I do appreciate the refresher.

But focusing on the original question, and purely in the theoretical here, as I can only imagine ever coming across this type of situation in real life, nor would the slight difference truly be detectable, but... the set up for the question...

Let's say that you have a piece of art (a flat plane of focus) that you wanted to capture all of the detail you could gather in that piece... and you had studio lights that were adjustable so that you could have a lens with the aperture set in the sweet spot for the lens...

Would a longer shutter produce more detail than a short one?

[KJ Smith]

If you have the aperture set to the "sweet spot", then your shutter must be set to get the correct exposure period.

Now, just to throw a kink in, a lower ISO would produce more detail. at least in film terms.

Think about the speed of light.

Kevin

[Dobick]

Originally Posted by KJ Smith

If you have the aperture set to the "sweet spot", then your shutter must be set to get the correct exposure period.

Now, just to throw a kink in, a lower ISO would produce more detail. at least in film terms.

Think about the speed of light.

Kevin

While this is true if you don't have lights that you can control, I specifically added in a detail about lights being controllable light source. What I did not specify that I should have is continuous lighting so that you don't negate the shutter speed with the flash speed.

So my question (and I think if I read it right, the original question from Jeff) still stands, do you get more detail with a slower shutter speed?

[KJ Smith]

Does not matter if you have a continuous light source or a flash, you still have to set the shutter speed to get the correct exposure for the aperture you selected.( sweet spot)

Think about the speed of light, you are getting everything there is to get no matter how fast the shutter speed is ( as long as the exposure is correct ).

Go to the sports section and look at the speeds Bob Levy shoots at and the detail he gets.

Detail is dependent on the resolving power of the lens and recording ability of the film/sensor not the shutter speed.

Kevin

[CaptainTom]

Does not matter if you have a continuous light source or a flash, you still have to set the shutter speed to get the correct exposure for the aperture you selected.( sweet spot)

I'll have to disagree with this statement.
If I am shooting in a darkened room, the duration of the flash acts as the "shutter speed" for my exposure. Whether I set my camera shutter speed to 1/4 or 1/250 doesn't have an effect because the duration of light is so short. Your ETTL flash computes the duration of the flash (your flash always flashes at the same intensity) according to the light conditions and the aperture you have selected.

If there is ambient light it is a different story and that's a different thread.

[KJ Smith]

This is basic photography.

If you chose a given aperture, then you must use the correct shutter to allow the right amount of light onto the film/sensor.

To much light and you lose detail in the whites, to little and you lose detail in the dark areas.

Note that he said continuous light source.


Tom, in your example, it must be a totally dark room, otherwise you will pick up ambient light, just like when you drag your shutter.

But you have proved our side. Shoot in a totally dark room and see what kind of detail you get, provided that you use the correct exposure.

Kevin

[venchka]

Originally Posted by KJ Smith

Wayne,

Check #1

Kevin

1. As sensors shrink, so does the depth of field.

Kevin,

I have been using online DOF calculators a lot lately. Here's what I have found. I'll use 105mm @ f/11 because I own 105mm lenses for 4x5, 6x7 & 35mm. Hyperfocal distance first & nearest point in focus second.

4x5: 32.3', 16.7'
6x7: 53.6', 27.1'
35mm: 106.9', 53.5'
40D: 168.6', 84.3'

Perhaps I should have said: "Smaller sensors have less depth of field for a given focal length and aperture."

This is the other reason why I'm holding out for a full frame DSLR. I won't give up FOV or DOF. When I put a lens on a body I want to know that it will behave the same regardless of the body I'm using.

[KJ Smith]

OK, had to go back and read this old thread.
http://www.texasphotoforum.com/forum...ad.php?t=38243

Still makes my head hurt.

Kevin

[venchka]

Grinning. Me too. That's why I stick to proven formats.