[Ouroboros]

Staff note: This post may contain affiliate links, which means Pentax Forums may earn a small commission if a visitor clicks through and makes a purchase. If you would like to support the forum directly, you may also make a donation here.

---

Just in case any of you are having trouble calculating what kind of magnification you can get from extension tubes and lenses, check out this page! It has a calculator where you can enter the lens measurement and tube length and the magnification amount is calculated for you! Easy!

Macro Extension Tubes & Close-up Lenses

[Adam]

Nice- thanks for sharing!

[Lowell Goudge]

interesting, but...

the calculator gives you distance to the sensor which is really not correct, it should calculate to the front element, which is close to the optical "center" of the lens.

If you look up the formulas, you can do the same calculations yourself.

the other thing to note is that while useful to have one point, what is more useful is to have the range of magnifications and working distances.

You do this by first working out the native lens extension of your lens, based upon close focusing distance, or simply by measuring it.

I have built the calculations for both close up lenses and extension tubes into my PDA in a spreadsheet.

It took some work to get the first one, but then it was copy and past for all others, and now I have it for all my lenses, in my pocket when I am out shooting.

[rhodopsin]

interesting, but...

the calculator gives you distance to the sensor which is really not correct, it should calculate to the front element, which is close to the optical "center" of the lens.

If you look up the formulas, you can do the same calculations yourself.

the other thing to note is that while useful to have one point, what is more useful is to have the range of magnifications and working distances.

You do this by first working out the native lens extension of your lens, based upon close focusing distance, or simply by measuring it.

I have built the calculations for both close up lenses and extension tubes into my PDA in a spreadsheet.

It took some work to get the first one, but then it was copy and past for all others, and now I have it for all my lenses, in my pocket when I am out shooting.

"Close (minimum) focusing distance" (MFD) and "working distance" (WD) are confused here.

MFD (and distance on lens scales) is the distance from film/sensor to subject, while WD is measured from front of lens to subject.

You've got me really dreading when I finally have to take some (any) math classes at our community college. I'm a very part time student.

[newarts]

The math is simple and easy to do:

Magnification_change_by_extension = Extension_change/Focal_length

Example: a 90mm lens is extended 30mm:
magnification changes by 30mm/90mm = 1/3X

Working_distance_change_by_extension = Focal_length(1/new_magnification - 1/old_magnification)

Example: A 50mm lens' magnification is increased from 1/3x to 1/2X:
working distance change = 50mm(2-3)= - 50mm

Dave

PS expressing magnification as simple fractions like 1/2, 1/3, 1/5, etc makes estimating working distance changes easy since the fractions are inverted in the calculation.

For example, say a 50mm lens has a natural magnification of 1/5 (pretty common) what's the working distance change if the magnification is increased to 1/2 by extension tubes?

Working distance decrease = 50mm(5-2) = 150mm

using extension tubes, each time the number on the bottom of the magnification fraction changes by one, the working distance changes by one focal length.

Since the only thing wrong with my K1000 is the meter is dead, I've been considering buying one of these things ((( New Cap Pinhole Lens Gold - 35mm DSLR/SLR Pentax Camera - eBay (item 400033876924 end time Dec-18-10 19:37:21 PST) ))) and using extension tubes in order to give it multiple focal lengths. With a purely mechanical shutter, no worries about battery wear to keep the shutter open for long (or very long) exposures.

But I have no idea about figuring exposure times for such a situation, nor how much the exposure would change with each different combination of extension tube segments. This one has an f/ stop of 168 at 42mm (just mounted on the camera body) with a 0.25mm pinhole diameter. I'm sure that there are other pinhole sizes possible, though I'm not sure why any particular photographer might need more than one, but each change, or tube set with different lengths of segmennts would change the resulting numbers.

And though I do have a couple of old hand-held exposure meters (does either one use a battery still made? I don't know yet), just what would be an appropriate set of values to use it at in order to begin these calculations for the pinhole?

(The only reason I got a 71 in algebra on the community college's pre-registration evaluation quizes was that I ended up treating the questions as logic problems, and ignored the unreasonable answer choices.)

[newarts]

F stop for a pinhole lens is (pinhole distance from sensor)/(aperture diameter)

Exposure time is proportional to (f stop) squared so is also proportional to (distance from sensor) squared.

Dave

[krp]

How do I figure out what the magnification on my lens is to begin with? I have a 28mm lens that focuses about 5-6 inches away.

[newarts]

How do I figure out what the magnification on my lens is to begin with? I have a 28mm lens that focuses about 5-6 inches away.

1) Lens specifications will often state magnification (see mfg's data on-line.)

2) Take a photo of a ruler and figure out the magnification:
Magnification = (Millimeters shown on photo)/(sensor width)

Sensor width = 23.6mm for K-x

3) Use the manufacturer's closest focus distance (distance from sensor to subject) - on the lens barrel or the smallest distance on the focusing ring. You can estimate magnification, M, using closest focus distance, D, and focal length, F:

M ~ (D/2F-1) - SquareRoot((D/2F-1)^2 -1)

Example: a 28mm lens has a closest focus distance of 5.5" (140mm) so D/2F = 2.5
M ~ (2.5-1) - SquareRoot( (2.5-1)^2 -1) =0.38X

Dave

[RioRico]

Using a pinhole on extension is an exercise in patience.
I have a pinhole-in-a-cap that is effectively 45/180. Oy.

Knowing the magnificaton of a setup, exposure is found with:

EA = NA * MAG+1

where EA is effective aperture, NA is nominal aperture, and MAG is magnification. So a 3x setup with the lens nominally at f/11 gives an effective aperture of 11 * 4 = f/44 or four (4) f-stops difference.

I could run off the formulae for magnification as a function of focal length and extension, but why bother? I just obtained (for US$40) a cute little M42 BELLOWSCOPE pre-mounted with a Steinheil Culminar 105/4.5 lens. The Bellowscope has two rails. One rail is marked with magnifications for a 50mm lens (up to 12x), the other for a 105mm lens (up to 3x). No calculations needed, except for exposure (if my TTL metering broke down).

Side note: No lens can focus closer than its focal length. No matter how much extension is behind a 50mm lens, it still won't focus closer than 50mm / 2 inches.