FX vs DX Lens

[PJM]

I decided to take a few test shots today to help me understand the difference between FX and DX lenses. I used both a 70-300 mm FX and 70-300 mm DX on a DX body. Both were zoomed to their max of 300 mm. The shots were also taken from a tripod so that the distance to the target did not change. The FX lens on the DX body (#1) appears to have a slightly higher magnification than the DX lens on the DX body (#2). From what I think I know I would expect them to be the same.
#1
Test1-2 by Peter Martin, on Flickr

#2
Test1 by Peter Martin, on Flickr

Is this because the focal point on the FX lens does not match the focal plane of the DX body vs a FX body? Normal variation in lenses? Something else?

Thanks.

 

[480sparky]

If the focal point wasn't 'matching the focal plane', the image would be out of focus.

I'd say the difference is due to 'rounding off' the focal lengths of the two lenses. A 70-300 lens won't be 70.0000000000 to 300.0000000000. One might actually be 66-285, while the other is 72-313.

 

[Derrel]

There is roughly a 10% allowable difference between stated maximum focal length and actual focal length. I have seen several tests of 300mm prime lenses and it is not uncommon to have a 300 millimeter lens that tests out at 285mm.

 

[Derrel]

Focal length is measured and tested with the lens focused at Infinity. On many zoom lenses as the lens is focused closer and closer, and the barrel extends to Focus closer, it is pretty common for focal length to drop. About two models back now, the Nikon 70 to 200 m m f / 2.8 VR zoom dropped to around 143 mm at its closest focusing distance. Many people call this Focus breathing.

 

[Derrel]

If you were to repeat your test at Infinity, you might find that the two lenses are closer in focal length.Looking at your example photos, I get the idea that you made the test around 20 feet or so, and in this case there might be a fairly high amount of focus breathing, causing one lens to appear to be quite a bit different in length from the other lens.

 

[zulu42]

Focal length is measured and tested with the lens focused at Infinity. On many zoom lenses as the lens is focused closer and closer, and the barrel extends to Focus closer, it is pretty common for focal length to drop. About two models back now, the Nikon 70 to 200 m m f / 2.8 VR zoom dropped to around 143 mm at its closest focusing distance. Many people call this Focus breathing.

I've noticed a prime lens changing focal length during focusing, is this also focus breathing? It's kind of a hassle when focus stacking.

 

[Derrel]

Yes it is pretty common with prime lenses as well. For example my Tamron 90mm F2 .8 macro is right around 90 mm at Infinity but at maximum close-focusing distance focal length drops to 73 mm or so as I recall.

Certain internal Focus macro lenses such as the Canon 100, f2.8 EF version 2 use internal element movement and do not have hardly any Focus breathing as the lens is focused closer and closer. The first version of this lens had a tremendously extending Barrel, and had a lot of loss of effective aperture as the lens grew considerably longer as it was focused closer and closer, as does my Tamron 90.

 

[PJM]

It's amazing the things you have no idea about when you get into this.

Thanks everyone for the replies.

 

[PaulR70]

It is my understanding you have to take in the sensor size, crop sensor or full sensor. Take the focal length and multiply by 1.5 for the FX lens. So the FX lens at 300mm on the DX body would be around 450mm. That is 300 x 1.5, and crop sensors may be different, it may be a factor of 1.6 instead of 1.5. My Nikon D3400 crop sensor has a factor of 1.5 for a FX lens. To be honest all my lenses are DX so I have not shot with an FX lens myself, I am just sharing what I have learned from Nikon and from others.

 

[RVT1K]

It's amazing the things you have no idea about when you get into this.

Thanks everyone for the replies.

As with many things..the more you know, the more you find out you don't know.

I've often looked at knowledge as a door. When you learn something, you open the door. Except that door always leads to a hallway lined with new doors to open...

 

[480sparky]

It is my understanding you have to take in the sensor size, crop sensor or full sensor. Take the focal length and multiply by 1.5 for the FX lens. So the FX lens at 300mm on the DX body would be around 450mm. That is 300 x 1.5, and crop sensors may be different, it may be a factor of 1.6 instead of 1.5. My Nikon D3400 crop sensor has a factor of 1.5 for a FX lens. To be honest all my lenses are DX so I have not shot with an FX lens myself, I am just sharing what I have learned from Nikon and from others.

A 300mm lens is still 300mm, whether it's on an FX body, a DX body, a smart phone, or even an 8x10 view camera. MMs are mms, all day long.

What changes between formats is the field of view.

 

[Dao]

It is my understanding you have to take in the sensor size, crop sensor or full sensor. Take the focal length and multiply by 1.5 for the FX lens. So the FX lens at 300mm on the DX body would be around 450mm. That is 300 x 1.5, and crop sensors may be different, it may be a factor of 1.6 instead of 1.5. My Nikon D3400 crop sensor has a factor of 1.5 for a FX lens. To be honest all my lenses are DX so I have not shot with an FX lens myself, I am just sharing what I have learned from Nikon and from others.

Keep in mind that focal length is the optical property of the lens regardless of the recording medium. So as Sparky said, 300mm is a 300mm because that is the lens optical property, not the recording medium.
Let's put it this way, a medium size adult jacket. When a 8 years old kid wear that *Adult Medium Size* jacket, the jacket feels like a XXL jacket. When a 6 foot 5 body builder wear the same jacket, it feels like a XXS size. You see, it does not matter who wear that jacket, it is still a Adult Medium size jacket. Not XXS nor XXL.

So why others kept saying the crop factor and need to multiple the focal length with the 1.5 or 1.6 crop factor? Well, it only matters if you are used to the 35mm format film system (or Full Frame). If you never use the full frame or 35mm film format, the crop factor actually does not mean too much to you because you have nothing to compare with.

So if you look at the viewfinder on your camera and set the lens to 300mm, the Field of View of the 300mm on your camera is what matter. It does not mean much to you if I tell you the field of view is similar to a full frame camera with lens focal length of 450mm.

 

[PaulR70]

It is my understanding you have to take in the sensor size, crop sensor or full sensor. Take the focal length and multiply by 1.5 for the FX lens. So the FX lens at 300mm on the DX body would be around 450mm. That is 300 x 1.5, and crop sensors may be different, it may be a factor of 1.6 instead of 1.5. My Nikon D3400 crop sensor has a factor of 1.5 for a FX lens. To be honest all my lenses are DX so I have not shot with an FX lens myself, I am just sharing what I have learned from Nikon and from others.

A 300mm lens is still 300mm, whether it's on an FX body, a DX body, a smart phone, or even an 8x10 view camera. MMs are mms, all day long.

What changes between formats is the field of view.

I agree with you, it is the field of view that changes, 300mm lens is still a 300mm lens but the field of view would be equivalent to a 450mm if it is an FX lens on a DX body. I guess I didn't explain myself very well, I did not intend to say that it changes to focal length of the lens.

 

[zulu42]

It is my understanding you have to take in the sensor size, crop sensor or full sensor. Take the focal length and multiply by 1.5 for the FX lens. So the FX lens at 300mm on the DX body would be around 450mm. That is 300 x 1.5, and crop sensors may be different, it may be a factor of 1.6 instead of 1.5. My Nikon D3400 crop sensor has a factor of 1.5 for a FX lens. To be honest all my lenses are DX so I have not shot with an FX lens myself, I am just sharing what I have learned from Nikon and from others.

A 300mm lens is still 300mm, whether it's on an FX body, a DX body, a smart phone, or even an 8x10 view camera. MMs are mms, all day long.

What changes between formats is the field of view.

I agree with you, it is the field of view that changes, 300mm lens is still a 300mm lens but the field of view would be equivalent to a 450mm if it is an FX lens on a DX body. I guess I didn't explain myself very well, I did not intend to say that it changes to focal length of the lens.

Hi PaulR70, keep in mind the "crop factor", or field of view you're speaking of - also applies to your DX lenses. It's not just FX lenses that will have the "cropped" field of view on a DX body.

 

[480sparky]

And this is why I wish the 'crop factor conversion' idea would disappear from the face of the earth. Erase it. Delete it. Send it to the Trash Bin. Eradicate it. As if it never existed.

Back in my film days, there was no such thing as a 'conversion factor' to compare lenses between 135, 120 and 4x5 formats. I never heard of a number to multiply (or divide by) when changing from 35mm to 6x4.5 format, or to 6x7 format, or to 4x5 format. I never had to 'convert' the 80mm lens of my Mamiya 645 to 'the equivelant of __mm on my 35mm camera". Nor convert the 150mm on my 4x5 to 'the equivelant of __mm on my RB67 camera". A 50mm was standard on my 35mm bodies, 80 was standard on my 645, 105 standard on my RB67 and 150mm standard on a 4x5.

I understand why the manufacturers created the 'conversion' factor. It was a marketing tool to help sell fledgling digital SLRs to an uneducated populace. The bulk of that populace that had grown up with one format: 35mm. Everyone and their uncle owned a 35mm film camera. Most carried a 50mm 'standard' lens. Many purchased 28mm wide-angles and 135mm telephotos. So that was 'the gold standard' back then.

Then along came digital. But the sensors were smaller than a 35mm frame. So the manufacturers needed an easy way for those transitioning to digital to 'relearn' how focal length related to FOV. "Conversion factor' seemed like the perfect choice. And it was. At least back then.

But today, we have an entire generation that has never even SEEN a 35mm film camera, let alone understand focal length, sensor/film plane size and how the two relate to FOV. So today, the 'conversion factor' has royally muddied the waters. Not to mention the internet, and it's ability to spread incorrect information that will be accepted as gospel.

And next, the assumption (and even firm belief with some) that the 'conversion factor' applies to other properties of the lens, such as aperture and minimum focus.

I say it's time to deposit 'the conversion factor' concept into the dustbin of history. Let it reside the Hall of Useless Technology, next to ice picks, buggy whips and 8-track tapes.