Zoom Lens Question

[droad]

I'm still a true beginner. I've always assumed that photos taken with the same camera of an object at the same distance with two different lens set for the same focal length would yield a similar sized photo of the object. However in a test today I'm finding that isn't true.

Please ignore my photo quality as I was just trying to test the concept. I used a Nikon D7200 on a tripod. The subject is a box sitting at 55" from the focal plane. I used a Nikon 55-300 DX lens at 300mm and a Tamron 16-300 DX lens at 300mm. As you can see in the resulting photos of the box is very different.

I'm trying to figure out why that is.

 

[Derrel]

As I recall, the CIPA industry standard allows for about a 10 percent variation in lens focal length specification versus actual, measured focal length; I have seen a few test results over the years, including one in which a 300mm lens was actually around a 285mm lens. Also, some lenses suffer from focal breathing, a term for when a lens's focal length changes with focused distance. Focal lengths are measured with the lens focused at Infinity; on many lenses, as the focus is set closer thanh Infinity, the focal length shortens

 

[photo1x1.com]

Hi and welcome to the forum.
This is totally weird. While Derrel is correct (of course he is ). This is just waaaaay more than 15% - more like 100% +. I´ve looked at the exif data because I thought you mixed up something. But true, both are shot at 300mm. So my guess is: either you are trying to make fun out of us and changed the image in Photoshop, keeping the exif data from a totally different shot . Or there is something wrong with one of your lenses. I don´t think the camera has digital zoom, does it?

 

[astroNikon]

I'd run the same test for both lenses at 55, 100, 150, 200, 250 and 300
just because those images are so far off I'm curious where the lenses are off from each other.
Also make sure when you go to 300 that it is fully extended and it's just not shown on the lens number.

 

[BrentC]

The first image looks like a crop.

 

[Dave442]

I expect that is just because you have focused on something very close. Try again focusing on subjects at infinity as Derrel suggested.

It is very common for zoom lenses to change their actual focal length as they focus close to their closest focusing distance (the focal length may become shorter as you focus close to the lens closest focus distance).

 

[BrentC]

I expect that is just because you have focused on something very close. Try again focusing on subjects at infinity as Derrel suggested.

It is very common for zoom lenses to change their actual focal length as they focus close to their closest focusing distance (the focal length may become shorter as you focus close to the lens closest focus distance).

By that much though? That is a huge difference.

 

[Dave442]

I expect that is just because you have focused on something very close. Try again focusing on subjects at infinity as Derrel suggested.

It is very common for zoom lenses to change their actual focal length as they focus close to their closest focusing distance (the focal length may become shorter as you focus close to the lens closest focus distance).

By that much though? That is a huge difference.

This thread... Nikon 70-200 II -- effective focal lengths at "200mm" shows differences between the two VR versions of the Nikon 70-200 lens, similar to what the issue the OP has.

 

[KmH]

Superzoom lenses (10x or more zoom range) are the worst, and usually have several other optical aberration issues, like soft focus, barrel & pincushion distortion at various ranges of the total zoom range.
The Tamron lens the OP used has an 18.75x zoom range - a super, super zoom lens.

IIRC Nikon's other superzoom lens the 18-200 mm when zoomed out to an indicated 200 mm, but used at it's closest focus, only delivered about 135 mm worth of actual zoom.

 

[droad]

First of all - thank you guys for your help. I've spent more hours than I should admit reading and learning on this site so please know that all your time has been of a great help to me.

I tried close photos again today (~60 inches with both lenses at 300mm) and then an outside shot at about 100' with both lenses at 300mm.

I see the same major difference with the close photos. For the distance photos there is a very minor difference. So I have to assume that inside the Tamron there is some action taking place on close photos. Although I set the lens for 300mm it must be adjusting internally to a shorter focal length based on the distance to the subject. At long distance all seems as it should. So between Dave442 and Darrel I think I know what is going on.

Why am I worried about this?? I suspect like many folks starting on a Nikon I have the 18-55 and 55-300 DX lenses. That combination provides the focal lengths I need but results in some lens switching. I was curious whether something like the Tamron 16-300 would yield similar image quality in one lens with a wide range of focal lengths. I rented the Tamron and while testing some close up shots I noticed the issue that prompted my question.

These photos are all right out of the camera.

 

[dxqcanada]

... so the topic is the comparison of image magnification at minimal focus distance of different zoom lenses set at the same focal length ?

 

[droad]

That is correct. In this case at a long distance two 300mm zooms yield a comparable level of magnification. However at close distances there is a significant difference in the magnification.

 

[Derrel]

That is correct. In this case at a long distance two 300mm zooms yield a comparable level of magnification. However at close distances there is a significant difference in the magnification.

VERY COMMON!!!! The close-up magnification ratio of many lenses is often listed in the manufacturter's specifications; listing this information allows the astute user to ascertain if a lens loses a lot of focal length at MFD, or Minimum Focusing Distance. For example, at 60 inches, the maginifcation ration for the Nikon lens is MUCH higher than the magnification ratio for the Tamron lens.

 

[Bebulamar]

It's called focus breathing. While focused at a distance object lens focal length do varies but not by much. When focused at close object many lenses (especially the newer ones) shorten their focal length. Newer lenses tend to do this more because of the need of not moving the lens too much when focusing for rapid AF.

 

[droad]

Thank you guys for the help. Maximum Reproduction Ratio, Magnification Ratio, Focus Breathing - lots of stuff to learn! I've seen the ratio listed in product specs but didn't know what it meant. I now see the Nikon lens has a ratio of 1:4 and the Tamron lens has a ratio of 1:2.9. That appears to be the difference that mattered in this test.

I see a number of articles and videos on the web on this subject. I'll have to give them a look.