Aperture Sweet Spots for Macro Lenses

So I was reading a post about a member's first pictures with his new D70 and some mention of being overstopped was made. The poster indicated that the lens being used produced greater depth of field at a wider f stop (f16 instead of 27 or f32). I would like to hear opinions on where the sweet spot is for the Nikon 105mm micro. I shot the lens on my first trip last week at f22 a lot and forgot all about this concept. Did I miss the optimal usage point for the lens? Where does one enjoy the greatest depth of field when photographing in the land of macro?

The DOF gets better as the aperture decreases, so F/32 will have better DOF than F/16. However, for the sensors in the current batch of DSLR's, diffraction causes the image resolution to decrease for apertures smaller than ~ F/16 so the overall sharpness decreases. The consumer camera sensors have about 1/2 the pixel pitch. That's why they limit the aperture to F/8 or so. It's not because they can't make smaller holes.

Here are a couple of good references:

http://www.photo.net/learn/optics/lensFAQ

http://www.photo.net/learn/optics/lensTutorial

The DOF gets better as the aperture decreases, so F/32 will have better DOF than F/16. However, for the sensors in the current batch of DSLR's, diffraction causes the image resolution to decrease for apertures smaller than ~ F/16 so the overall sharpness decreases. The consumer camera sensors have about 1/2 the pixel pitch. That's why they limit the aperture to F/8 or so. It's not because they can't make smaller holes.

This was the same for film - being digital and sensors hasn't changed this much.

My experience over time has shown that with the 105mm Nikkor lens, F22 is about as far as you can go before you start losing sharpness and focus. The most consistent results are at F16. However I have seen some differences between individual lenses, so it is worth taking example shots of the same subject at the various F-stops to see where your camera/lens combination works best.

M

Living here is Santa Barbara there are many photographers professionally trained at the Brooks Institute in Underwater photography. They are taught to limit your F stop to macro to F16 due to the effects of diffraction. Every now and then I try this but I always find I go back to F22 for my macro. The results seem better to me. Smaller than F22 is definitely worse when I have tried it.

Diffraction calculations are very easy. Do the calc and look at the diffraction spread for violet vs red light. Compare this spread to the size of a pixel in your camera, and you can "quantitatively" determine the maximum aperture you can shoot. For the current crop of DSLR's, this happens to be F27.

Qualitatively, Paul has determined that his shots look sharpest at f16 - and that's fine too.

So to answer the original poster's questions: You may get LESS depth of field at f16, but the portion of the subject that IS in focus may look sharper.

This holds especially true when you use a teleconverter by the way - sometimes you are forced to shoot as low as f8 to get sharp photos.

Cheers

James

Diffraction calculations are very easy.  Do the calc and look at the diffraction spread for violet vs red light.  Compare this spread to the size of a pixel in your camera, and you can "quantitatively" determine the maximum aperture you can shoot.  For the current crop of DSLR's, this happens to be F27.

 

 

James

Aperture is nomally defined as the acutual size of the openning. Diffraction limits the minimum aperture or the largest f-number N which is the ratio of the focal length to the diameter of the openning, usally written f/N.

F/27 is well into the region where the red light is diffraction limited. Have a look at the first reference I cited. The diffraction limited Modulation Transfer function MTF for any lens is MTF < 1/(wavelength * N) . If you plug in around 700nm for red light and a sensor size of 22mm, you'll find that the theoretical max resolution is 1400 line which is less than the 1500 that the 3000 lines of pixels can do. So F/22 is borderline and F/27 is too small.

Roger that.

Cheers

James

I knew you fellas would have a handle on this. The diffraction issue makes sense to me now and I would never have dreamed that you all would provide such an in depth explanation.

I had an opportunity to take some intraoral photos at work today with my S2 for a cosmetic dental case and I took time to try the different settings. The f16 does actually work "better" than f27 and with your explanation, it all makes sense.

Too bad I didn't take just a second while in Bonaire to consider this. Instead, gripped with the excitement that overcomes all of us with our new toys, I just kept snapping shots and blaming the indian, not the setting of the arrow for what seemed like consistently soft focus results.

Thanks, the next trip will produce real winners!

Too bad I didn't take just a second while in Bonaire to consider this.  Instead, gripped with the excitement that overcomes all of us with our new toys, I just kept snapping shots and blaming the indian, not the setting of the arrow for what seemed like consistently soft focus results.

Whoa now.... I wouldn't go that far. I've have many shots in my portfolio taken at f27 and smaller that are acceptably sharp. Maybe you should post some images.

I posed the fstop issue to a well known pro at DEMA, and he said in most circumstances you should shoot at the smallest aperture you can when shooting macro. I tend to agree - It can be so difficult to frame/focus your subject UW and a small aperture will save you more often than hurt you. Shooting land macroon a tripod is a different ball game, however.

Thanks for a very useful information. I will try a few different F-stops next time I try the camera. I guess there is something new to learn everyday.

Diffraction calculations are very easy.  Do the calc and look at the diffraction spread for violet vs red light.  Compare this spread to the size of a pixel in your camera, and you can "quantitatively" determine the maximum aperture you can shoot.  

Very easy???? I would hate to see what you classify as 'quite difficult' or 'very hard'. :shock:

FWIW, I'm sort of happy to shoot f/27 and certainly f/22 on my Tamron 90mm/10D, and haven't really noted softness at f/32 although I'm sure it's there and tend to avoid shooting stopped all the way down.