is 24mm prime the same as 24mm on a 18-105 lens

[amolitor]

Also, I think she's referring to the entrance pupil as the easy thing to find, and it is pretty easy to find, it turns out. It's precisely the point about which rotation produces no parallax errors. So, rig up a setup that's sensitive to parallax error, and start rotating the camera around different points. It shouldn't take more than a few minutes.

 

[ratssass]

"The Beginner's forum is for asking basic technical photographic questions about things like shutter speed, aperture, ISO, white balance, metering modes, focusing modes."

 

[Gavjenks]

Also, I think she's referring to the entrance pupil as the easy thing to find, and it is pretty easy to find, it turns out. It's precisely the point about which rotation produces no parallax errors. So, rig up a setup that's sensitive to parallax error, and start rotating the camera around different points. It shouldn't take more than a few minutes.

Yes, but to do this:

1) You need a rig that lets you pivot the whole camera system at any point you desire, which is non-trivial (something like a piece of wood with a slot you can slide a nut and bolt across with the tripod mount on one end and a ruler taped to it, but that might not be stable enough for millimeter precision. Might need aluminum or something.).
2) You have to re-perform this entire procedure after focusing, because the entrance pupil could very well change position with respect to easy reference points on the lens' physical body as focusing occurs. So you have to spin the thing around at different pivots until you get no parallax, then position the front node and the object, then focus it, then double check you didn't mess everything up by spinning everything again, and then you probably did mess it up, so you have to now reposition a bit, and refocus again and again until it converges to a limit. For both lenses. It's a converging loop of effort.

And in order to observe the parallax, you're going to want a nice clear well defined object (similar to the pegboard thing) in front of you anyway, that is sensitive to parallax. So I submit that's it's simpler to just use that parallax-relevant object to see if the focal lengths are the same in the first place, without worrying about any of the parameters of the lens. Just move each lens+camera combo until the nearer pegs are equal in the frame, then measure relative parallax in the photos.

Requires less building of stuff (namely the more difficult of the things to build: the precision bracket) and less effort for the same answer.

"The Beginner's forum is for asking basic technical photographic questions about things like shutter speed, aperture, ISO, white balance, metering modes, focusing modes."

This is all directly relevant to the OP's question. The goal is finding a sufficiently easy to do method that a beginner like him/her could follow to answer whether his/her two specific lenses are actually equal focal lengths at the same marked values.

 

[amolitor]

You appear to be sufficiently muddled at this point that I am going to stop attempting to unravel the mess.

 

[Gavjenks]

It's not that complicated. In order for front node-->object to be sufficient in the way you explained before, both lenses have to be in focus, as you said, right?
But the position of the front node (often) changes when you focus using the lens' normal focusing controls.
So unless you are focusing by freelensing or a bellows, you're going to have to re-figure out your front node position after you focus. And then when you reposition it at the correct distance from the object, you will throw yourself out of focus. Thus, back and forth and back and forth.



If you freelens it, it's not a problem, but then you need some way to make sure that the sensor and the lens are perfectly aligned, or you can't use the photos to compare framing, obviously. How do you do that? Additional rigs, additional complexity.
If you use a bellows and macro rail, it's easy, but the beginner photographer would probably not own one and would thus have to go unnecessarily buy something to answer the question.

 

[amolitor]

The beginner doesn't care about nodal points or entrance pupils, and should just compare apparent fields of view. You seem to be the only person who cares much about nodal points.

 

[Gavjenks]

What? From the OP:

at 24mm (or 36 actually) is the 24mm prime, 24-85 @ 24mm and the 18-105 @ 24mm have the "exact" same coverage ?

Obviously, this question implies "...when positioned at the same location." If you aren't at the same optical location, then you would expect the coverage to change even with the exact same lens, much less two different lenses.

Location as it turns out, is tricky, because a camera is not a single point in space, and this thus requires doing things like measuring nodal points (if following Helen's suggestion), or some other type of effort to make sure that if the lenses WERE equal focal lengths, that the setup would guarantee equal coverage. You can't just "compare apparent fields of view" off the cuff without worrying about this at all. You will likely get an incorrect answer.

 

[Braineack]

i didnt realize this was thinkgeek.com

 

[amolitor]

The OP was clearly looking for an approximate answer, cf "number of bricks" despite the use of the word "exact".

It's turtles all the way down, and the actual exact answer for the OP is "certainly not, it probably depends on how far away the subject is, among other factors". You can unpack the optics pretty much forever, getting increasingly detailed answers, but the OP doesn't care. The OP just wants to know if the same number of bricks are going to show up more or less.

 

[Gavjenks]

Not my fault that the OP (accidentally or not) asked a very non-beginner-y technical question in a beginner's forum.

Also, it's not like it's just a completely trivial nuance. The actual focal length can differ from the stated one by a LOT sometimes. upwards of 10% for some manufacturers. Earlier in this same thread, Dao actually gave some examples of Tamron lenses where it was off by 14% and 17%. You can potentially fit several bricks in that difference. Worst case scenario, I wouldn't be surprised if you could fit many dozens of resolvable bricks in the difference, depending where you stand.

Edit: And the difference between two different lenses could be even more extreme. If one overstates its FL by 15% and another one understands its FL by 15%, then the difference in actual focal lengths for the two lenses that claim to be at the same zoom could be more than 30%! At such a difference, I would not say that they are the same at all, even in the most approximate of senses.

 

[Braineack]

Suppose I pitched my camera up to photograph a brick, and were asked how the brick came to be there.

 

[table1349]

"The Beginner's forum is for asking basic technical photographic questions about things like shutter speed, aperture, ISO, white balance, metering modes, focusing modes."

You have to keep in mind that some of the later posters in this thread are beginners in the field of comprehension. There fore they go off on wild unsubstantuated tangents that have nothing to do with the orignal question being asked by the OP. And of course some of them just like to see their words in type whether it has anything to do with the topic or not.

Helen was just trying to keep all the misinformaiton from being propogated.

 

[Gavjenks]

There fore they go off on wild unsubstantuated tangents that have nothing to do with the orignal question

How is it irrelevant that focal length might not be the value written, when somebody's question is "are written focal lengths equal between lenses?"

Also, YOU YOURSELF were one of the people to originally bring this up:

think I'll test for myself too.

I'm assuming I'll have to move the position of the camera based on the length of the lens, ie, have the lens opening at the same distance from a wall versus having the sensor at the same location.

FOV is going to be slightly different for all lenses. There is nothing that says 24mm lens has to be exactly 24mm and not 23.99mm or 24.01mm. If I can find it I have a chart of Canon and Nikon lenses and their exact measures specifications. I haven't looked at it for 4 or 5 years so it may take a while.

When you talk about it, it's relevant, but when somebody else does, it's a wild unsubstantiated tangent? Interesting.

 

[amolitor]

There's a difference between "look, the marked focal length ain't always 100% right" and getting all muddled up by nodal points, entrance pupils and so on.

 

[table1349]

Soft kitty,
Warm kitty,
Little ball of fur.
Happy kitty,
Sleepy kitty,
Purr, purr, purr



Now go to your room and quietly build youself a cold fusion reactor Sheldon or we will have to call your mommy down in Texas and tell her you are broken again.
:mrgreen:


FYI. The OP got it and reported it back to the forum.
http://www.thephotoforum.com/forum/...-prime-same-24mm-18-105-lens.html#post3048225

That might have been a clue right there that the OP's question had been answered.