90, 100, 105 macros not "constant f/2.8"?

[knyghtfall]

this is a very interesting and informative article - didn't know so much about focal lengths and f -stops until I read this... and even after reading through the entire thread and also additional research on wikipedia, I'm quite sure I'm still quite muddled on the issue!

But I think the whole confusion with how the f stop changes with close focusing is the confusion of the f-number of the lens (e.g. the f2.8 notation we get after the focal length of the lens) and the effective f-stop that the lens is deployed at. Lowell did quite a good job clarifying, but if you're a math person you can look at how the effective f-stop changes when the object is not far from the lens, at: F-number - Wikipedia, the free encyclopedia, under the section titled "working f numbre"

Anyway, if I may venture a comment to Will's latest post, it might not be true to say that a macro or close focus lens is like many prime lenses in 1 housing, primarily because the focal length of the lens remains the same - its simply that the lens is engineered so that you can use it to focus very close to your objective. Its true in the sense that you can then use the lens for different purposes, i.e. normal photography and macro photography, but that's not very different from say, using a 50 mm f1.2 for bokeh photography and street shooting (when stopped down appropriately).

back to your original post, whether or not you're not looking at a macro lens, you've lotsa options for primes in the 90 - 105mm focal range, many of which have macro or close focusing ability: pentax (K 100mm and 105mm, M 100mm/2.8, M 100mm /4 macro, various A, FA and DFA 100mms), sigma 90mm, tamron 90mm, voigtlander 90mm, etc, and prime lenses in this range are pretty good, so you've no dearth of choices here!

[axl]

Now, did I get this correctly?:
If I take picture with 105/2.8 Sigma macro at f2.8 foccused at infinity in M (exposure) mode and achieve ccorrecct exposure, then picture of the same subject, in the same light with the same lens/camera combo in the same M setting and at f2.8 BUT foccused to 1:1 would come underexposed?
Did I get it right?

Thanks
Peter

[Lowell Goudge]

Originally Posted by axl

Now, did I get this correctly?:
If I take picture with 105/2.8 Sigma macro at f2.8 foccused at infinity in M (exposure) mode and achieve ccorrecct exposure, then picture of the same subject, in the same light with the same lens/camera combo in the same M setting and at f2.8 BUT foccused to 1:1 would come underexposed?
Did I get it right?

Thanks
Peter

Peter, that is correct.

you are projecting the same amount of reflected light from the subject over a bigger area therefore it gets darker.

The same holds true for photo enlargement and also slide projection, the bigger the enlargement the dimmer the image.

Try this yourself perhaps with a 50mm macro on a copy stand, and you will see it.

[axl]

Originally Posted by Lowell Goudge

Peter, that is correct.

you are projecting the same amount of reflected light from the subject over a bigger area therefore it gets darker.

The same holds true for photo enlargement and also slide projection, the bigger the enlargement the dimmer the image.

Try this yourself perhaps with a 50mm macro on a copy stand, and you will see it.

Thanks for confirmation Lowell.
Unfortunnately, I don't have 50maccro, nor copy stand
But I have another question or few:
How apparent is this exposure shift with non macro lenses. Say something like 40/50 mm normal lens? Does it mean that even with these lenses I have adjust exposure when close fucusing while using M mode to achieve correct exposure? Does in Av mode cammera adjust for this or it still thinks that effective f stop is 2.8 therefore it underexposes?
Thank you for this, it seems I still have a lot to learn.

BR
Peter

[Lowell Goudge]

Originally Posted by axl

Thanks for confirmation Lowell.
Unfortunnately, I don't have 50maccro, nor copy stand
But I have another question or few:
How apparent is this exposure shift with non macro lenses. Say something like 40/50 mm normal lens? Does it mean that even with these lenses I have adjust exposure when close fucusing while using M mode to achieve correct exposure? Does in Av mode cammera adjust for this or it still thinks that effective f stop is 2.8 therefore it underexposes?
Thank you for this, it seems I still have a lot to learn.

BR
Peter

Peter

exposure shift from Infinity to minimum focus is very little perhaps 1/10 of a stop, and is managed by the camera's internal meter.

I have not tried to measure this but I would be amazed if it amounts to more than 2-3 points of grey scale value when looking at the image in an image editor. Note for perfect exposure a grey card should give you a value of about 120 in a pentax camera, and at normal contrast, each stop is roughly 45 grey scale (for the first 2-3 stops either side of this value)

Note that since digital cameras shoot in steps of 1/3 of a stop, you would not notice this unless you measure in an image editor

As I said earlier, the exposure meter would manage these minor changes and even major ones with a macro lens.

What I don't know is how p-ttl flash manages this but it may just assume the F Stop reported by the lens is correct, and since it measures the light from the preflash, and ambient light of the scene, it probably does just fine. Someone working in macro and with an "A" lens may be better on this point

As for learning, it never seems to stop

[pentaxman]

One easy way to see this effect is to put a macro lens on your camera and set the lens to aperture priority and set the aperture to the largest aperture. (smallest number). Focus on a target at infinity and change the focus to the minimum focus, you will see the shutter speed change to slower shutter speeds as the focus changes to closer focus.

Originally Posted by axl

Now, did I get this correctly?:
If I take picture with 105/2.8 Sigma macro at f2.8 foccused at infinity in M (exposure) mode and achieve ccorrecct exposure, then picture of the same subject, in the same light with the same lens/camera combo in the same M setting and at f2.8 BUT foccused to 1:1 would come underexposed?
Did I get it right?

Thanks
Peter

[axl]

Lowell what do you mean by value 120for grey card?
What is the 120???
TIA

[Lowell Goudge]

Originally Posted by axl

Lowell what do you mean by value 120for grey card?
What is the 120???
TIA

Axl

In an image histogram, in your photo editor, aside from the graph, it should give you the mean or meadian value, i.e. exactly where the peak of the histogram is located.

Histograms go from 0 to 255.

Pentax sets up the metering in their cameras to attempt to make a correctly metered spot come out to a value of 120, therefore, using a grey card or grey surface, if you meter off that surface, there should be a very high spike in the histogram at a value of 120.

If you then, with the same aperture, double or halve the exposure , the peak will move by about 45 higher for slower shutter and 45 lower for higher shutter.

I established these things a long time ago by imperical measuremens to understand exactly what the histogram shows and how things change with changes in contrast settings.

I hope this answers your question

[nater]

I have a question related to this - is there any practical difference between using a macro extension tube on a non-macro lens of the same focal legnth as a "true" macro lens?

[Lowell Goudge]

Nater

the answer is yes.

lenses are always designed to do specific things. Macro lenses are designed to have thier best sharpness etc close up, not far away as a normal lens would (or might be)

Additionally, macro lenses are designed to be truely flat field, so that when at right angles to a surface, everything corner to corner is sharp and in focus. not all regular lenses are flat field.

Macro lenses are designed to be stopped down, to between F8 and F16 for maximum depth of field and sharpness (before difusion starts) depending on focal length.

[Class A]

Will,

Originally Posted by WMBP

So any one of these three lenses (Tamron 90, Pentax 100 or Sigma 105) could be used simply as a fast telephoto lens, right?

Yes. Make sure to include the Sigma 70/2.8 EX into your considerations. Extremely sharp and nicely doubles as a portrait lens.

Originally Posted by WMBP

I gather that there's a switch of some sort on the lens that causes it to switch from normal telephoto mode to macro mode. In normal telephoto mode, I can get a "normal" f/2.8. Or to put it differently, I could use the Pentax 100 (say) to take a picture of somebody 25 ft away at f/2.8 and get a result very similar to the result I'd get if I took the same picture with my Pentax 50-135 f/2.8. Right?

First, I'd like to clarify that I meant to say that Lowell did not misunderstand you. AFAIC, he understood you correctly and gave the correct answers from start to finish.

Note that some macro lenses have a switch but this is for controlling a focus delimiter that prevents the lens from using the close focusing range when you do normal shots. Otherwise it could take the lens a long time to search through its complete focusing range in order to obtain focus.

Other than that, with a standard macro lens you do not need to switch from telephoto to macro usage. As has been said before the f-ratios are accurate for infinity focus. The closer you focus, the more will there be a difference between the set f-ratio and the effective f-ratio. Somewhere I've read you can distinguish between two f-ratios in such scenarios: One that represents the DOF and the other the light loss. Not sure if that makes complete sense but it seems that some macro/camera combinations seem to adapt the f-ratio in close focusing situations (perhaps showing the light-loss f-ratio) while others do not make that change explicit and just keep displaying the same f-ratio.

To reconfirm: When you use a macro in non-macro situations, it will behave just like a regular lens and the images will look the same.

[axl]

Originally Posted by Lowell Goudge

Axl

In an image histogram, in your photo editor, aside from the graph, it should give you the mean or meadian value, i.e. exactly where the peak of the histogram is located.

Histograms go from 0 to 255.

Pentax sets up the metering in their cameras to attempt to make a correctly metered spot come out to a value of 120, therefore, using a grey card or grey surface, if you meter off that surface, there should be a very high spike in the histogram at a value of 120.

If you then, with the same aperture, double or halve the exposure , the peak will move by about 45 higher for slower shutter and 45 lower for higher shutter.

I established these things a long time ago by imperical measuremens to understand exactly what the histogram shows and how things change with changes in contrast settings.

I hope this answers your question

I thought you were talking about that. I just wasn't sure.
Thank you for confirming. I'll try to remember the 120 value.
Thank you Lowell.

BR
Peter

[Class A]

Originally Posted by axl

I'll try to remember the 120 value.

To be precise, it should be 128 (the middle of an 8-bit spectrum).

[Lowell Goudge]

Originally Posted by Class A

To be precise, it should be 128 (the middle of an 8-bit spectrum).

yes and no

there was a thread about this and it is below the middle

[newarts]

All lenses exhibit similar behavior, even simple, thin lenses.

For a fixed aperture opening the light intensity projected on the image plane changes with the distance from the lens to the image plane.

Image_plane_to_lens_distance = Focal_length/(1+magnification)

Therefore the effective f-stop for light intensity for a simple, thin lens is:

F-number_effective=F-number(1+magnification)

For "normal" photography magnification is usually quite small, like 1/100 or less, so the effect is ignored in discussions.

Dave in Iowa