DOF-F number, Focal length, MFD & Sharpness

[Anandarajkumar]

Dear Friends,

I was having a discussion with a friend/hobbyist photographer. So at the end got confused...  I am just starting out with my doubts and requesting seniors/experts to clear them, please.

1. According to my understanding, anything above the MFD the lens will autofocus and from that focussing point the DOF will be according to the F stop chosen.
2. According to my friend's view, at particular F stop, the DOF will be maxium at a given particular focal length only and for a given F stop, we need to choose the focal length based on the DOF. In other words, for a given lens and F stop, we need to choose the distance of the subject according to the DOF.
3. So if I choose not to get confused by all these calculations, can I simply use the MF instead of AF?

I felt like we will end up in too much calculation before we click...

Or I am wrong? Seeking experts suggestions....

[Ashim]

DOF depends on magnification and f-stop. Magnification depends on focal length and subject distance. You can get 1:5 magnification with a 50mm lens and a subject distance of 50cm and with a 180mm lens at 100cm subject distance (assumed, as an example). The DOF at f/4 will be same for both the lenses at these distances and, therefore, at the same magnification. Mind you, the BG will look very different for a 50mm lens and a 180mm lens with the same subject distance and f-stop.

One needn’t do complex calculation for judging DOF though programs are available, including a Java application that runs on mobile devices, to calculate DOF on the field where you input the FL, subject distance and the f-stop. In prime lenses, you have DOF scales printed to help one judge DOF. An approximate idea of DOF comes from experience and a fairly accurate one by pressing the DOF button.

Hope that was of some help.

[lightwave]

Quoting > "We need to chose the focal length based on the DOF."
I think you have the horse by the tail here. Focal length, Aperture and Distance to subject affect the DOF. DOF does not affect any of them. DOF is an effect not a cause. The online calculator is here: http://www.dofmaster.com/dofjs.html

What needs to be done is, to click a certain picture, set the distance to subject (some scenarios like wildlife and birding you might not be able to change it too much), set the focal length (zoom in or out) to get the composition. Again with Primes you may nt be able to alter this step. Once these two parameters are finalised, in all probability, if you are experienced at what you are doing, you have already set the F-stop number because you don't need to look through VF to know the above two factors. You can judge with the eye. If not, then set the F-stop now to get more DOF or shallow DOF and click the pic. Basically three parameters are important. What order you set them and if can even alter them depends on your skill, your gear and your experience.

[Anandarajkumar]

Ok. Thanking you for those explanations which entered somehow into my dullhead.  Now let me be more practical. All these things and presumptive calculations I feel were of much use in Film photography. When we go digital, why cant we simply follow trial and error method?

So far this is what normally I do:

I choose a subject. I assume and choose an F stop based on lighting and for some guessed DOF; zoom the lens for compostion. Then based on the cam's light meter, I just adjust the shutter speed. If still could not bring it to optimal level, then will change the F stop again; then I click. I dont like the result, I change the F stop and repeat the exercise.

Do I go logical ?

[lightwave]

When we go digital, why cant we simply follow trial and error method?

Well as long as you are learning on the way and number of trials and number of errors keeps going down its fine. Even if not so what? You are answerable to no one but yourself.

[Image]

I choose a subject. I assume and choose an F stop based on lighting and for some guessed DOF; zoom the lens for compostion. Then based on the cam's light meter, I just adjust the shutter speed. If still could not bring it to optimal level, then will change the F stop again; then I click. I dont like the result, I change the F stop and repeat the exercise.

Its easier to change the spark plug on a stationary car. Point is that some subjects won't wait for you for your trial and error success.

[Anandarajkumar]

Doctor, got that. To be on the happier side, my no. of trials are minimizing!! And I go nearby though not exactly reach the result, I want.

Ramji, I got your point, esp., in nature and action photography where subject is not in our control.

But the point I wanted to understand is: My friend suggested - we have to know the DOF for a particular f stop and choose our Focal length! That is where I got confused. I felt that was like altering the feet for the footwear.

Now I see this clear.

[Hankosaurus]

Hello Anand.

But the point I wanted to understand is: My friend suggested - we have to know the DOF for a particular f stop and choose our Focal length! That is where I got confused. I felt that was like altering the feet for the footwear. Now I see this clear.

There is no single "the DOF for a particular F stop." At any given lens to subject distance and aperture, DOF will be different for lenses of different focal lengths (or a zoom set at different focal lengths).

It might be instructional to go to the link Doc recommended, download the free DOFMaster software and work with it on your PC for awhile. You can see and change all the parameters and examine what effect each of them has with respect to the others, at least in a numerical sense.

http://www.dofmaster.com/dofjs.html

When we go digital, why can't we simply follow trial and error method?

You can. So can the film imagemaker. Only the cycle time for feedback is much longer. Some keep notes when experimenting with film, to compare later with negatives, slides, or prints. Now with digital we can just "chimp" and try again, if time permits. But it's far better to know how to get it right in the first place.

If one is to elevate himself above hit and miss automatic photography, at some point he will have to learn the underlying fundamentals of imagemaking, and thereby gain actual control of the camera.

Even the automatic shooter benefits by understanding the foundational principles. He is better able to understand how automatic cameras "think," and can anticipate when they will think wrongly. So, he can redirect the machine's efforts into a more productive path.

[ISO]

I assume and choose an F stop based on lighting

Let ISO take care of lighting. You choose F stop for DOF.

[Anandarajkumar]

Thanks Henry and Mukund.
But Mukund, my 400D allows me upto ISO 200 for decent noise free images. Body limitation ...

[Hellwrath]

Thanks Henry and Mukund.
But Mukund, my 400D allows me upto ISO 200 for decent noise free images. Body limitation ...

I have gotten decent shots with my 350D at ISO 800 . You need a good NR method in Post processing, check out Nik's Dfine 2.0.

[bippz]

Wonderful discussion and great learning thread.
I thank you all for helping us understand the  basic facts.
Regards.

[Anandarajkumar]

Thanks Henry and Mukund.
But Mukund, my 400D allows me upto ISO 200 for decent noise free images. Body limitation ...

I have gotten decent shots with my 350D at ISO 800 . You need a good NR method in Post processing, check out Nik's Dfine 2.0.

Thanks about that HW.

[sasikgera]

Only Lightwave and Hennry gave correct answers.