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Am I misunderstanding something?

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Barmax

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I just responded to another thread regarding the focal length on DX and FX lenses. I always thought Nikon converted the DX number to reflect the 35mm equivalent.

For example, I shoot with the 18-200 DX 75% of the time, I also have the 35mm/f2 and had the 50mm/f1.8.
  • When looking through the viewfinder using the 18-200 at 50mm, everything looks normal.
  • When looking (returned it, too long indoors) through the viewfinder using the 50mm everything was larger than normal, around 75mm using the 18-200
  • When looking through the 35mm lens everything appears close to normal. Just above 50mm on the DX lens.
So why does everyone upconvert the DX lens focal numbers which seem to already be adjusted by the manufacturer?

As an FYI, I do not do drugs (social drinker), perfect vision with my contacts on.

Maybe someone can explain.

--
D200 with 18-200 kit lens
 
Barmax said:
I just responded to another thread regarding the focal length on DX
and FX lenses. I always thought Nikon converted the DX number to
reflect the 35mm equivalent.
Click to expand...
No.
Barmax said:
For example, I shoot with the 18-200 DX 75% of the time, I also have
the 35mm/f2 and had the 50mm/f1.8.
  • When looking through the viewfinder using the 18-200 at 50mm,
everything looks normal.
Click to expand...
Where are you focused? Unless you're focused at infinity, the actual focal length of the 18-200mm will be less than what you see on the lens barrel. This especially noticeable at the 200mm position, where the 18-200mm is actually more like 135mm at close focusing distances.

This is a common trait among lenses that have an "internal focus" design.
Barmax said:
  • When looking (returned it, too long indoors) through the viewfinder
using the 50mm everything was larger than normal, around 75mm using
the 18-200
  • When looking through the 35mm lens everything appears close to
normal. Just above 50mm on the DX lens.

So why does everyone upconvert the DX lens focal numbers which seem
to already be adjusted by the manufacturer?

As an FYI, I do not do drugs (social drinker), perfect vision with my
contacts on.

Maybe someone can explain.

--
D200 with 18-200 kit lens
Click to expand...
--
http://www.pixelfixer.org
 
I notice this when I focus on subjects between 6 and 10 feet. The 50mm prime is definitely much longer than zooming into 50mm on the 19-200.

So it's an optical illusion I guess.
--
D200 with 18-200 kit lens
 
The FOV of a given lens is defiend at the infinity setting.

Various lens desings will shorten the focal length when focusing closer w/o letting you know in the exif data, the 18-200 is among them. It is no real 200mm when you focus close. Also the new IF macro designs are like that. so the 60mm is no real 60mm if you focus real close.

This has nothing to do with FX/DX !!!
--
Regards
Nikko Ono
 
Barmax said:
I notice this when I focus on subjects between 6 and 10 feet. The
50mm prime is definitely much longer than zooming into 50mm on the
19-200.

So it's an optical illusion I guess.
Click to expand...
Hmm... I wouldn't call it an optical illusion, since an optical illusion is where you look at one thing, but you think you're seeing something else!

It's just a "quirk" of that type of lens design.
Barmax said:
--
D200 with 18-200 kit lens
Click to expand...
--
http://www.pixelfixer.org
 
I had the same question when I was buying my dSLR - if I look through the viewfinder on my D40 (I also tried this on a Canon, I think) with a 50mm lens it looks "normal" (which I checked by opening both eyes). Just like when I look through my old 35mm camera with a 50mm lens. Guy in the camera store told me it was because the viewfinders on the dSLRs are so much smaller, which seems to make sense.

When I mount an OLD 50mm lens on my D40 it looks the same as when I use the kit lens at 50mm.
 
I really can't tell what is the matter with the lens. When I set my D80 with 18-135 on 50 i see perfectly as with my eyes. Same size picture. The door knob is the same through the VF and without it. Using both eyes, i am sure it is so. I like that.

I want to get the 50mm f1.8 and you say it is longer? How much? 18-135 is relatively same in terms of focus, and the 50mm infinity is very close to 50mm close up. On what mm should I put 18-135 to get the view of a 50mm f1.8?
--



How many megapixels you need from a camera:
http://www.bhphotovideo.com/FrameWork/charts/resolutionChartPopup.html
 
I guess the proof is in the pudding, since I have no 50mm lens anymore I will post pics using the 35mm lens and the 18-200 DX lens at 35mm.

The test will be of a subject 6 feet away I'll post the JPGs with EXIF data.

My prediction

Picture 1 - taken with 35mm lens
Weird, looks normal, like a 50mm lens (this is where the crop factor comes in)

Picture 2 - 18-200 DX lens at 35mm
Ahh, wide angle

Picture 3 - 18-200 DX lens at 50mm
Looks normal, Wow for some reason it looks like the same shot with the 35mm!

--
D200 with 18-200 kit lens
 
When I had the 50mm 1.8 it definitely was not normal, the 50mm lens on the D200 was more like 75mm using the 18-200DX.

Wow! same as the crop factor.

So the crop factor should only be applied to non-DX lenses.

I bought the 35mm/f2 which is very close to normal, 52mm if you whip out the calculator.

--
D200 with 18-200 kit lens
 
NikKi Ono said:
The FOV of a given lens is defined at the infinity setting.

Various lens desings will shorten the focal length when focusing
closer w/o letting you know in the exif data, the 18-200 is among
them. It is no real 200mm when you focus close. Also the new IF macro
designs are like that. so the 60mm is no real 60mm if you focus real
close.

This has nothing to do with FX/DX !!!
Click to expand...
I'd like to second that...

A few years back I spent a couple of weeks (!) communicating with Nikon service, wondering if my 18-70 which had come with the D70 'kit' was perhaps defective, or mis-calibrated as it seemed longer at 70mm than other lenses, including the venerable 70-200... I did several tests, took comparison shots (indoors ...focussing at 12ft) and sent them to Nikon. Funny thing... the Nikon techs themselves couldn't figure out what was causing the discrepancy, and two of them actually sent me conflicting messages... each telling me to ignore what the other one was saying (!) Lol :) They apprently disagreed as to whether or not I was seeing 'the DX effect'. Meanwhile, based on my own further tests (working with friends who had identical camera/lens combos) I concluded that this was a calibration issue with that particular lens design --neither a defective unit, nor anything to do with DX/FX.

So, yes, it might come as a surprise to some, as it did to me, but not every zoom lens design follows the same calibration curve. But in the final analysis that means nothing to me. When working with a zoom lens I care more about what I see in the viewfinder than whether the camera says it is 55mm or 70.

--
John
http://www.JChristopherGalleries.com
 
1. A "normal" lens is one whose focal length is equal to the diagonal of the imaging plane. On 35mm film, that's actually about 42mm (a 50mm lens is cheap/easy to make and close enough to be called "normal"), and on a DSLR with a 24mm x 16mm sensor it's about 28mm.

2. If the image you see through your viewfinder is life-size, it doesn't mean the lens is "normal". If you mount a viewfinder magnifier the viewfinder image would seem bigger even with the same lens, so really you're just assessing viewfinder magnification when you do that.

--
http://www.pixelfixer.org
 
No, you ARE misunderstanding.

Nikon do not print equivalent focal lengths on their DX lenses. They print the actual focal length. It so happens that a DX lens at 50mm has a narrower field of view than an FX lens at 50mm, but only because the image circle is smaller. A narrower field of view will make the DX lens at 50mm on a DX sensor look like a 75mm full frame lens on an FX sensor. However, both lenses project the same size image on the sensor when both set to 50mm and the subject at the same distance.

Amy
 
What the mm number represents is the focal lenght of any lens, and that never changes, no matter if the lens is designed for FX, DX, medium format, 8x10, etc. a 50mm lens is always a 50mm lens, what changes is the field of view each format creates with a given focal lenght.

The "crop factor" is simply a formula designed for those who where used to what each lens produced in their 35mm SLRs so they could get an idea of what lens they would need to get the same frame they did with the old camera, so to get the "normal" view they now needed a 35mm, but in the same way that if they needed a "normal" view in a 8x10 format they would need a 300mm lens (IIRC), all this only because the size of the sensor or film and nothing else, so for nikon to "correct" the focal lenght in DX only lenses to the corresponding field of view of a film SLR it would only make everybody laugh at them or even sue them for false data, because the focal lenght never changed.

So the 35mm is a 35mm (that looks like a 50mm in the old film SLR or the brand new D3, or a 300mm in the 8x10, but still a 35mm), and the 18-200, 18-70, 17-55, 18-55 and any other DX only lense still has those ranges for focal lenght no matter what size of sensor or film is used with.
 
All wrong.

A true 50mm on a DX lens is the same as 50mm on a FX lens. The only time the effective FL is given is with compacts, because they all use different crop factors.

What you are seeing with the 50mm is part of the lens design of the 50mm. Its not a true 50mm, it really ends up being rounded down to 50mm. As you focus the FL changes too, so a 50mm ends up being a 53mm-57mm (not the exact numbers, couldn't find those). Add to that, I wouldn't really trust a zoom lens markings to be absolute science.
--

 
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