[Tuner571]

After reading through this forum it seems I have been doing something wrong. When I want full detail in an image I usally set the f stop to its highest setting(f/22) to get the most depth of field. Normally this works just fine, but is this the correct way to get full detail.

Sorry if this a stupid question but I'm self taught and this is the way I have been doing it for the past year.

Thanks for any help!

[Gooshin]

small aperture (high f number) will result in more things IN focus.

larger aperture (smaller f number) will result in less things in focus before and after what you are focusing on, this is called depth of field.


being IN FOCUS and SHARP are not one and the same.

each lens usualy has a sweet spot at a specific aperture size where it will give the SHARPEST result of everything that is in focus.

usualy for lenses on the wider side this is between 4 and 5, for telephoto lenses this is usualy around 8-12

at f22 you are losing sharpness, but you are gaining focus of everything in your frame.

[Tuner571]

Thanks for clearing that up for me, I was wondering why in automatic mode it was using a aperture of 11. I thought that in order to get a crisp shot you needed to use the smallest aperture the lens had.

[travis_cooper]

The reason you are losing sharpness at f/22 is because of diffraction. Diffraction is caused by bending light. The light will give you the sharpest picture if it doesn't have to bend in order to make it into your sensor. As you move up in f-stop, meaning close down your aperture, the light has to bend more and more to get in, this makes the image not as sharp. There are some sites that try to tell you where, for your lens, diffraction is just too bad to be worth it. Usually this is around f/16 for most lenses I have looked up. I try not to go above an f/11. One cool thing that diffraction gives you though is the starts on lights. Because of the bending light, spots of light, like light poles, start giving you that star affect, and the more aperture blades your lens has the more points on the star. So sometimes a high f-stop can be used in your favor.

[Tuner571]

Thanks for the input, much appreciated!

[dws1117]

Thanks for asking that question and many thanks for the terrific answers. It has helped put a lot of thing in focus.

[Lowell Goudge]

just a quick note about defraction.

Defraction is caused by light bending around the edges of the diaphram (apature).

It is present at all f stops, and results in some loss of image quality at all stops.

What makes defraction bad as you stop down is the ratio of the perimiter of the diaphram (apature) to total area in the opening increases, causing the impact of defraction to become more and more noticable in the image.

The trade off between sharpness by reducing the apature, improved depth of field, and increased problems caused by defraction is at the f8-f16 range depending upon the individual lens.

[cardinal43]

The trade off between sharpness by reducing the apature, improved depth of field, and increased problems caused by defraction is at the f8-f16 range depending upon the individual lens.

Would I be correct then in assuming that you have no way of knowing this without spending a "fair" amount of time with each lens? I understand that you get a larger depth of field with a smaller aperture, but I'm still having a little trouble with sharpness vs focus. I have been spending some time reading Peterson's "Understanding Exposure" and trying to asorb what he is saying.

[Canada_Rockies]

Would I be correct then in assuming that you have no way of knowing this without spending a "fair" amount of time with each lens? I understand that you get a larger depth of field with a smaller aperture, but I'm still having a little trouble with sharpness vs focus. I have been spending some time reading Peterson's "Understanding Exposure" and trying to asorb what he is saying.

As a suggestion, wander out with your new toy, and find a field with trees and items on which you can focus at various distances. Take your tripod, turn off SR (Ruined my lunar eclipse shots because I didn't), turn off auto focus and set the image capture to RAW. You do not want jpeg conversion to foul things up for you.

With a digital camera the only loss will be time. At each aperture setting, at your widest focal length, take at least three images, focused way out there, 1/3 of the way out there, and maybe 2 or 3 meters away. Don't even try to rate the images in camera. When you have done this, take the images onto your computer screen. and examine them carefully. If you use the Pentax photo browser you can see all your EXIF data across the bottom of the screen. Somewhere in that bunch of images is the shot that says "for scenics, you want to use f/11 and focus at 10 metres."

You can repeat the exercise at other focal lengths another day. You will be trying to rate the best of several dozen images at each focal length. Eyes and brains get tired after a while. Pentax made this really easy with my M series 28/3.5. They marked f/8 with orange, marked one of the distance settings with orange. That was your no focus setting. Line up the orange and use the viewfinder only for framing. Worked like a whiz. Everything from 5 feet or so to infinity was in focus enough for an 8 x 10 inch print, and sometimes 10 x 15.

[Lowell Goudge]

Would I be correct then in assuming that you have no way of knowing this without spending a "fair" amount of time with each lens? I understand that you get a larger depth of field with a smaller aperture, but I'm still having a little trouble with sharpness vs focus. I have been spending some time reading Peterson's "Understanding Exposure" and trying to asorb what he is saying.

You can do as albert (canadian in the rockies) suggests, but many lens reviews had sharpness as a function of f stop.

Either way it does pay to know how your equipment performs, the only risk is that you get to fixated on the testing and loose sight of the actual process of taking pictures.

I have to question from time to time the effort people go to to attempt to prove their equipment is faulty.

[benplaut]

If I remember correctly (it's been a while), the general rule is to try and stay inside of two stops from the top and bottom of your range to have acceptable levels of diffraction.

[cardinal43]

Thank you, gentlemen.

[mattdm]

Would I be correct then in assuming that you have no way of knowing this without spending a "fair" amount of time with each lens? I understand that you get a larger depth of field with a smaller aperture, but I'm still having a little trouble with sharpness vs focus. I have been spending some time reading Peterson's "Understanding Exposure" and trying to asorb what he is saying.

You can also read charts and reviews to see what tests say.

Or, you can set the P mode to "MTF", and it will attempt to pick the sharpest aperture automatically using an internal lens database (given the constraints of the lighting conditions, of course).

[Canada_Rockies]

After reading through this forum it seems I have been doing something wrong. When I want full detail in an image I usally set the f stop to its highest setting(f/22) to get the most depth of field. Normally this works just fine, but is this the correct way to get full detail.

Sorry if this a stupid question but I'm self taught and this is the way I have been doing it for the past year.

Thanks for any help!

You can find easily what Pentax thinks is the lens' sharpest apertures by setting the program line to MTF and seeing what the camera wants to do. This won't find the happy medium between lens sharpness and depth of field, which is what my long winded test will do.

You made a comment in your last post about not using the largest nor the smallest aperture, and that's a pretty good rule of thumb. There was one in the 60's among journalists: "f/8 and be there" which is a pretty good rule, too.

[jeffkrol]

After reading through this forum it seems I have been doing something wrong. When I want full detail in an image I usally set the f stop to its highest setting(f/22) to get the most depth of field. Normally this works just fine, but is this the correct way to get full detail.

Sorry if this a stupid question but I'm self taught and this is the way I have been doing it for the past year.

Thanks for any help!

MTF