Calculating focal length and minimum focus distance

Laci Toth · Jul 5, 2020

Hi all,

I’d like to stack a Raynox 250 on a Super Takumar 200mm f4 lens for shooting macro.
I’d like to get to know how to find out the focal length and the minimum focus distance in order to be able to determine the depth of field. Raynox is a 8 diopter lens, the 200mm lens’ minimum focus distance is 2.5 meters.

ic-racer · Jul 5, 2020

1 diopter = 1000mm focal length

Adding the 8+ to your lens makes it 77mm.

Assuming 1/p + 1/q = 1/f

Where:
p = lens to subject in mm
q = lens to film in mm
f = 200mm

You can estimate your 'q' for whatever minimum focal distance you can get with you 200mm lens.

Then put 'q' calculated from above in the equation and solve for 'p' with a focal length of 77mm.

Your result 'p' would be a rough estimate of closest focal distance.

So, if you find your lens and the diopter focus to 1 meter then your DOF at f8 would be around 65mm using the equation below.
Where:
u = subject distance (1000mm)
f= focal length (77mm)
c = your minimum CoC size maybe 0.0025mm
N = f8

polka · Jul 6, 2020

Excuse me, when you put a 8d lens in front of the taking lens of any focal, when the taking lens is focused on infinity, your subject must be at 125mm, and if the minimal focus of your taking lens is "say" 1m, your subject must be at 111mm (8d+1d). And in both cases, the depth of field will be very shallow (I did not take the time to check your formula). Anyway, you cannot focus on a subject at 1000mm.
u

Laci Toth · Jul 6, 2020

So it’s still a question though I’ve to thank for the equations.
I don’t know about the lens to film distance because I do not have the lens yet.
I only know the 200mm lens’ minimum focus distance from the subject to the film is 2500mm. That’s what I know.
I also know now that the Raynox’s minimum focus distance is 77mm.

polka · Jul 6, 2020

Ok, with 2.5m minimal focusing of the lens, your subject must be between 125mm and 119mm (thus no use focus the lens), focus by moving the camera back and fro.
And at f8 you will get a DOF of 1mm (taking a COC of 0.025mm not of 0.0025mm - which is way too small). Most important : your field of view in a 24x36 image frame will then be 15mmx22mm (a magnification greater than 1 !).

Laci Toth · Jul 6, 2020

polka said:
Ok, with 2.5m minimal focusing of the lens, your subject must be between 125mm and 119mm (thus no use focus the lens), focus by moving the camera back and fro.
And at f8 you will get a DOF of 1mm (taking a COC of 0.025mm not of 0.0025mm - which is way too small). Most important : your field of view in a 24x36 image frame will then be 15mmx22mm (a magnification greater than 1 !).

Okay, thanks indeed! I think I skip to purchase the 200mm lens. I had a Raynox before and it’s fun and I used it on a 55mm lens but I wanted to increase the dof by getting a longer lens. As the dof won’t increase a lot I just leave it.

Dan Fromm · Jul 6, 2020

Laci, depth of field is determined by aperture and magnification. Focal length has nothing to do with it.

ic-racer · Jul 6, 2020

Focal length has nothing to do with it.

Unless you need to find the magnification...math is your friend.

Calculating focal length and minimum focus distance

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