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Kelpfish

F-stop and Shutter with D100 Digital

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Hi everyone, need a little input here.

 

I have been playing with my camera (D100) shooting flowers and bottle caps and have some questions relative to settings. I read somewhere that the higher the f-stop when shooting macro (22, 28, 33, whatever) the more likely you will get either noise or a type of digital circles of confusion. I am finding that I can get good exposure at 250th shutter at f22 and higher with no problem. But is that optimal for image quality. I assume the balancing act for DOF and negative space focus exists with digital but have yet to hear much feedback on this topic relative to the limits of the camera. What settings do you recommend while using your macro set up? Is there a max f-stop one chooses to use to optimize image quality? And does a higher shutter impact anything other than freezing the image and controlling light?

 

Thanks for the advice.

 

Joe Belanger

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The equivalent circle of confusion on a D100 is 1.5x smaller than on film. That means the smallest aperture (highest f-number) should be lower by a like amount. What the smallest aperture should be is open to debate. Some say f/22, others f/32 and still other would say f/11 something else. You aren't going to get any DOF at f/11 though! The only objective measure I can think of for maximum f-stop is the value that sets diffraction effects equal to the circle of confusion. That's dependent on the color of light you are talking about but it is roughly f/27 on the D100. It is my policy to shoot the D100 at or below f/27. f/22 would probably be better.

 

When you shoot macro underwater. Your exposure will be determined entirely by strobe power and aperture. Shutter speed will be a non-factor.

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I read somewhere that the higher the f-stop when shooting macro (22, 28, 33, whatever) the more likely you will get either noise or a type of digital circles of confusion.

 

Interesting description. It's not really noise or "digital circles of confusion". It's diffraction. It's taking all that light coming into your lens and bending it so it goes through an ever shrinking hole size as the aperture decreases (higher f number).

 

I am finding that I can get good exposure at 250th shutter at f22 and higher with no problem. But is that optimal for image quality.

 

Depends on how you define quality. If you define narrowly as sharpness/resolution, then no, f/22 isn't the best. Each lens varies in performance, but most lenses are sharpest at the midpoint of their aperture range + 1 stop. As you move away from the midpoint, sharpness decreases. It is not a uniform move, nor is it consistent from lens to lens. You often hear the call "f8 and be there". If I care about nothing else, I start at f8 and adjust from there.

 

Now, in macro photography and large format photography, we stop down, usually to least f/16 and as much as f/32. This is to get the required DOF to make a scene a "quality" shot. A tack sharp image at f/8 where half the subject is blurry is of little value. The most common technique in macro photography is to stop down as much as necessary to get the DOF you need and no more. Stop down too far and the whole image starts losing sharpness due to diffraction.

 

I've never had diffraction issue at f/22, so this is my rule of thumb for aperture starting point for macro.

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The problem with using very small f-stops in macro photography is diffraction, and the problem is balancing that with depth of field. I've seen advice from top photographers to never go beyond an effective aperture of f/16 when shooting at magnification levels of 1:1. (Note that the effective aperture can be different from the marked f-stop when shooting macro.)

 

That said, I personally tend to ignore this advice and use the f-stop that delivers the DOF I need. I suspect that the tolerance level is very dependent on what you're doing with the image later. Blowing up and printing large would presumably reveal diffraction problems that would be invisible in an image printed smaller, or used on the web.

 

As to shutter settings, underwater you will usually be completely dependent on strobe illumination for macro, so the shutter speed is pretty much irrelevant, provided of course that it isn't higher than the max flash synch speed. The real shutter speed, in effect, is the duration of the strobe flash, which will be something on the order of a few thousandths of a second.

 

There are times when it's possible (and desirable) to get ambient lit background water recorded on the image when shooting macro, and for this you'll need to experiment with much lower shutter speeds (and wider apertures).

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Effective aperture is what is indicated by the D100. It needs to understand the lens you're using (which isn't always the case). Curious that such advise would be for effective aperture and not actual aperture, though. The difference between then is based on lens and light geometry and diffraction with be controlled by the actual aperture only.

 

The difference between effective and actual aperture is roughly:

 

effective = actual * (1 + magnifcation)

 

At 1:1 effective is one stop higher than actual. You lose 1 stop of light at 1:1. At 3:1 you lose 2 stops. Generally you move closer at higher magnifcations so the light loss balances itself out.

 

I pretty much hate the general rule of thumb thing. It's based on nothing but old generalized experience. A high quality lens designed to be shot wide open ( like a 400 f/2.8 ) will be sharp there. A macro lens most likely isn't intended to be used at f/4. The best aperture for a given lens is a compromise between optical aberration reduction and diffraction effects and each lens will be different. If you are a lens made for macro and believe you should shoot it at f/8 I think you're missing the point. I realize no one is advocating that. :?

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Not to confuse matters too much, but I always thought the diffraction becomes a problem w/ digital cameras when light is bent so it hits multiple pixels at a time. For example red light hits one pixel, while the violet light hits the pixel next to it.

 

Microlenses in digital cameras are there over the sensor to help w/ this, but they only work so well.

 

If you know your pixel size (6 microns for example) then you can calclulate the diffraction effects if you go back to your physics book. I think Craig has done it before and that was another way to get the f27 Aperture. It's the size of the aperture, not the f-stop that really matters, right?

 

Cheers

James

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Yes, but to get diffraction down below 6 microns would require you to shoot around f/8. I've read some people claim that the circle of confusion for any digital camera should always be equal to the pixel size and that you should shoot at f/8 for that reason anyway. That's ridiculous, of course, but some think this way. A perfect image without any DOF isn't worth taking.

 

My f/27 calculation was based on a 17 micron CoC and is equivalent in reasoning to why 35mm macro lenses are limited to f/32 (25-30 microns). Diffraction effects are dependent on wavelength, so the color light you use effects the computation. Red versus blue results in a full stop difference in results.

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