How does one calculate reproduction ratio...

Started Jul 31, 2010 | Discussions
nfpotter Veteran Member • Posts: 4,080
How does one calculate reproduction ratio...

How does one calculate reproduction ratio for a given focal length with a reversed lens?

I Beam Veteran Member • Posts: 3,494
Re: How does one calculate reproduction ratio...

nfpotter wrote:

How does one calculate reproduction ratio for a given focal length with a reversed lens?

My guess that this would not be an easy calculation to do. Using examples I have calculated reproduction ratios for lenses with extension tubes and it takes quite a few lines of manual calculation.

Anyway have look here:

http://www.jeffree.co.uk/pages/macro-lens-calcs.html

Probably the best way would be the empirical way. That is take photographs of a ruler and then estimate the reproduction ratio.

Regards

Paul

Brooks P Veteran Member • Posts: 9,183
It is really quite simple

It is best to reverse a lens of lesser focal length and attach it to the front of a longer focal length lens that is attached to the camera. You can buy a Macro coupler for this purpose fairly cheaply; Amazon lists a 52mm & 67mm coupling ring for $6.99 for reversing and attaching my 50mm f/1.8 to my 18-105 or 70-300mm lenses.

With the lens focused at infinity the magnification can be calculated by dividing the focal length of the reversed lens into the focal length of the normally attached lens. My 50mm reversed and attached to my 18-105 (zoomed to 50mm) would then produce a 1:1 ratio. With the 18-105 zoomed to 100mm the ration becomes 2:1 or twice life size.

Be aware that there will be significant vignetting and the working distance (distance from lens to subject) will be extremely short. I took the picture below a few years ago just holding my 50mm against the front of my 18-55mm lens. I only got the one frame as the fly fled the scene when the camera got so close; I think I was able to get the one frame only because the fly was lethargic from the cold. I had a lot of trouble taking this photo. The zoom wanted to collapse when I pressed the 50mm against the 18-55; trying to hold the 50mm against the normally mounted lens while also trying to manipulate the camera and hold the camera steady enough to keep the fly in the viewfinder as I moved closer was a real problem – but I only dropped the lens once. I was surprised at just how close I had to get before the fly popped into focus. The DOF was so small that keeping the fly in focus was a real problem. Maybe the noise of the shutter woke the fly up.

Cropped to eliminate vignetting

Nikon D50, Focal Length: 46mm, AF Mode: Manual, 1/160 sec - F/8, ISO 800

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While amateurs change the camera’s settings; many Pro’s prefer to change the light.

Brooks
http://bmiddleton.smugmug.com/

OP nfpotter Veteran Member • Posts: 4,080
Re: How does one calculate reproduction ratio...

Thanks for the replies, all.

The question I'm asking stems from this thread:

http://forums.dpreview.com/forums/read.asp?forum=1030&message=35865393

Some great macro's, in my opinion.

The person that responds first says " I struggle with 1:2, let alone 3:1". How do they know it's 3:1 reproduction ratio?

OP nfpotter Veteran Member • Posts: 4,080
Re: How does one calculate reproduction ratio...

Anyone else?

binary_eye Veteran Member • Posts: 4,290
Re: How does one calculate reproduction ratio...

nfpotter wrote:

The person that responds first says " I struggle with 1:2, let alone 3:1". How do they know it's 3:1 reproduction ratio?

Because the OP notes it was 3:1 in the subject?

I'm no expert on the subject, but I don't know of any formula to calculate magnification of a lens reversed directly onto the body. I think the magnification would change based on the optical design of the lens.

As has been suggested, the most accurate way is to reverse mount the lens in question and take a photo of a ruler with millimeter markings. Then, for Nikon DX, divide 23.6 by the number of millimeters that fit, horizontally, within the frame.

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