[alohadave]

I've actually compared the image size of my vivitar against my super-tak 85/1.9. The change in focal length is real

I think that you are going to need to show up examples.

[sigurdhu]

Dave,

I think the correct formula is: m = f/(d-f).

Sigurd.

The OP wanted to find the focal length.
Hence the formula for the focal length will be: f = md/(1+m)

Sigurd

[Lowell Goudge]

The OP wanted to find the focal length.
Hence the formula for the focal length will be: f = md/(1+m)

Sigurd

Actually omwojldmlike people tO post the results for their lenses

[RioRico]

Actually omwojldmlike people tO post the results for their lenses

Lowell, you should lay off the hallucinogens, they're producing strange results.

[Lowell Goudge]

Lowell, you should lay off the hallucinogens, they're producing strange results.

Actually you are seeing an apple product at it's best. My iPod has it's own interpretation of typing

[Pigeon]

It was good example, where 28-300 had shorter focus than 70-210.

[Lowell Goudge]

It was good example, where 28-300 had shorter focus than 70-210.

I quite agree, the issue is that all lenses are different, and what i was hoping was that we could some-how get this data available so people might have a chance to consider it as part of their purchase decision.

[normhead]

For convenience, it's pretty standard to only be concerned with focal length at infinity. Even assuming you're right and that focal length changes as you get closer to the lens, I'm not sure it's information I want to try and carry around with me. You can see the results through the viewfinder. If you don't get the result you like you switch lenses. That doesn't change because you've calculated a 15 focal lengths at different parts of the distance scale.

[RioRico]

For convenience, it's pretty standard to only be concerned with focal length at infinity. Even assuming you're right and that focal length changes as you get closer to the lens, I'm not sure it's information I want to try and carry around with me. You can see the results through the viewfinder. If you don't get the result you like you switch lenses. That doesn't change because you've calculated a 15 focal lengths at different parts of the distance scale.

Indeed. WYSIWYG applies. Eyeball the effects and adjust your position accordingly. Sure, such variance may be significant in technical / forensic shooting, where some greater degree of precision is required. But that work would likely call for use of something other than a consumer-grade varifocal lens, too.

[newarts]

One place where it matters a bit is for macro photography when the user cares about working distance. IF macro lenses may achieve high magnification by shortening their focal length.

This just means that something like the focusing distance at highest magnification should be in the specs list and the shopper should be aware of its importance.

For example the Tamron 180mm (IF) 3.5 macro lens' minimum focusing distance is 430mm at 1:1 magnification. That implies its focal length at 1:1 magnification is about 430/4= 108mm.

http://www.tamron.com/en/photolens/di_macro/b01.html

Tamron's 90mm IF macro lens is close to 72mm at 1:1 http://www.tamron.com/en/photolens/di_macro/272e.html

[Lowell Goudge]

where I think it matters is when looking at lenses and uses.

While I admit I had an error in the calculation of my 85mm, it is clear that some lenses do suffer rather badly in loss of focal length, and this has been confirmed by others on the post. the question is how quickly do you lose focal length?

what about some lenses like the 18-250mm lenses. We always tell people on the forum that 18 isn't wide enough in all cases, yet if you are in somewhat close, like a church interior, it may not matter because 18 is really 16 or 14. it is worth knowing at least where the limits are,

While I accept focusing near infinity it does not matter, focusing close, it might.

while the argument about field of view etc is valid, and you can reframe and move as you wish, movement is not always possible.

[sigurdhu]

As to the source of the formula,

M = f / (f-d) where D is the distance to the lens (S2) this is from the same wikipedia page, just a little further down than the formula 1/S1 + 1/S2 = f/F

Lowell,

The correct formulae here are:
M = f / (d-f)
1/S1 + 1/S2 = 1/f

Also note that S2 is the distance to the lens from the image plane.
Hence S1 is the distance to the lens from the object plane.
I think your d (or D) is approximately equal to S1.

[sigurdhu]

What about some lenses like the 18-250mm lenses. We always tell people on the forum that 18 isn't wide enough in all cases, yet if you are in somewhat close, like a church interior, it may not matter because 18 is really 16 or 14. it is worth knowing at least where the limits are.

Lowell,

Let us do the 18 – 250 mm lens as you suggest:
The main specifications at PENTAX NEWS release 2007 : smc PENTAX-DA 18-250mmF3.5-6.3ED AL [IF] state:
Minimum focusing distance: 0.45 m (1.48 ft) (0.45 m = 450 mm)
Maximum magnification: 0.28×

Using thin lens formulae:

m = S2/S1
S2 = mS1
S2 = 0.28S1

S1 + S2 = 450
S1 + 0.28S1 = 450
S1= 450/1.28 = 351.6 mm

f = mS1/(1 + m)
f = 0.28×351.6/1.28 = 76.9 mm
Quite far from 250 mm!


What would the total distance from the focal plane to the object plane be if
f = 250 mm and still m = 0.28×?

From the formulae above:
S1 = f(1 + 1/m)
S1 = 250(1 + 1/0.28) = 1142.9 mm

S2 = mS1
S2 = 0.28×1142.9 = 320 mm

S1 + S2 = 1142.9 + 320 = 1462.9 mm
So in this case the total distance from the focal plane to the object plane is more than 1 m longer than 0.45 m!

So what do you think?

[Lowell Goudge]

Sigurdhu

I think you should take shots at min and max focal length at a measured distance from the front element and look at the magnification factor , and use this to calculate the true focal lengths. With any of these lenses you don't know what FL maximum magnification is at but your first calculation suggests it is probably at maximu FL, as does the minimum focusing distance