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Old Jul 10, 2007, 10:43 PM   #1
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I have this lens about 2 months, I just discovered the pictures from this lens at 250mm are smaller than 210mm from the beercan, 200mm from 100-300mm APO and 200mm from Sigma 75-300mm APO. Any user has the same problem?
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Old Jul 11, 2007, 1:30 AM   #2
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if you use any maxxum based lens this is what the film sees..



the full size is 35mm..the red crop is how big the chip is compared to film...

take your lens mm x 1.5 for the true size...

so a 200 is really a 300


now if you buy an old wide angle be sure to figure out the #s first...you will have to get a wider lens to compensate


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Old Jul 11, 2007, 10:42 AM   #3
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MINOLTANUT wrote:
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if the tamron is newer it is calculated at the digital chip size...



the beercan looks closer because the chip isnt reading the full frame that film would...



so it is like a cropped pic...



an old 50 is like a 75...ect ect

As far as I know, even the new digital specific lenses still give you the same FOV as the made for film lenses. That is why we need a size like18mm, which gives you the equivalent film lens Field Of View of 24mm film lens. They can only be used on a digital camera.

KMdigital only lenses end with the"DT" suffix.The reason you can't use them on a film camera is thatthese lenses produce a smaller image circle than normal 35mm lenses. The image circle is only large enough to cover the APS-C size frame of current DSLRs. You can mount these lenses on film cameras, but you will get heavy vignetting (dark image corners). So essentially, these lenses are only suited for DSLRs.

The DT series also have digital specific coatings which are optimized for digital but I think the major reason they make these lenses is because they can get away with using less glass and other materials.

If "Isifs" is getting smaller images from this lens it is probably at close distances and because it's an internal focus lens. Focal lengthwill fluctuatewith focus distance with many of the newer, more compactdesigns because they happen to also beinternal focus lenses as is the18-250mm Tamron. It is a known issue that the loss of focal length at nearer distance of a lens with internal focusing is much larger than that of an ordinary lens.


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Old Jul 11, 2007, 10:48 AM   #4
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lsifs wrote:
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I have this lens about 2 months, I just discovered the pictures from this lens at 250mm are smaller than 210mm from the beercan, 200mm from 100-300mm APO and 200mm from Sigma 75-300mm APO. Any user has the same problem?
You are probably shooting at fairly close distances andif so this is quite consistent with an internal focus designed lens like the 18-250mm Tamron. The same thing happens to me when I compare my Tamron 28-300mm to my 75-300mm KMl lens. The KM produces a wider field of view when comparing both lenses at 300mm.
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Old Jul 11, 2007, 11:00 AM   #5
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Meanstreak is correct. It's because of the lens design. It causes your effective focal length to change with focus distance. If you focus closer to infinity, the angle of view you get should be closer to your other lenses when set to the same focal length (when on the same camera). The focal length specs assume infinity focus.

This kind of thing has nothing to do with film size versus sensor size, as you'll also get a narrower angle of view on digital compared to film with your other lenses, too. It's a lens design issue.



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Old Jul 11, 2007, 11:07 AM   #6
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This lens became 28-388mm for KM5D as stated on Tamron website http://www.tamron.com/news/35mm/18250di2.asp

I took the pictures around 10 feet away, by comparing the pictures from the Mininolta beercan 70-210, APO 100-300 and Sigma APO 75-300 all at 200mm, the picture's size are almost the same, but the picture from the Tamron 18-250mm at 250mm islook like 160mm from other lens. I will go to my friend's home to use hisSigma 18-200mm to see the differences.
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Old Jul 11, 2007, 11:15 AM   #7
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Read the last two posts.

It's the focus distance. ;-)

Effective focal length with that lens will change with focus distance. You won't get the same angle of view you get with most other lenses set to the same focal length if you're focusing that close. The focal length range in the specs assumes infinity focus. If you want something closer to what a different lens set to 250mm would give you, you'll need to focus on something closer to infinity (much further away).

Sorry, that's just part of the lens design (trying to cram that much focal range into a small, internal focus lens).

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Old Jul 11, 2007, 11:37 AM   #8
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P.S.

Just so you're clear on the difference. The focal length does not differ between a digital and non-digital lens if both are set to the same focal length.

The angle of view (apparent magnfication) will not differ between a lens designed for digital and a lens designed for film, if they are both used on a Digital Camera with a smaller sensor. They will both appear to be longer on Digital.

Any lens used on a digital camera with a sensor smaller than 35mm film will have a narrower angle of view (more apparent magnification) compared to the same focal length lens on a 35mm camera. It doesn't have to be designed for digital for that effect.

For example a 50mm lens on a DSLR with an APS-C size sensor will give you the same angle of view you'd have using a 75mm lens on a 35mm camera. It doesn't make any difference if that lens has a smaller image circle designed for digital or not (since the camera just doesn't use the entire image circle if the lens is designed for film).

So that users can have a better understanding of how that works, sometimes manufacturers will show you a "35mm equivalent" focal range that is 50% longer. That's because you need to multiply the focal length of a lens used on a DSLR by 1.5x to determine what lens is needed on a 35mm camera to give you the same angle of view. That goes for using *any* lens on a camera with a smaller sensor. The same thing applies to your non-digital lenses.

This has nothing at all to do with what you are seeing (why you have a wider angle of view compared to your other lenses set to the same focal length) when using them on your camera. It's because the lens design of that particular lens causes the effective focal length to change with focus distance (so it won't give the rated focal length unless you are focused to infinity).

Focus on something much further away and you'll see what I mean if you compare it to other lens designs.

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Old Jul 11, 2007, 1:19 PM   #9
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i guess the guy i asked last year at sonywas on crack then..:roll:
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Old Jul 11, 2007, 3:16 PM   #10
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MINOLTANUT wrote:
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i guess the guy i asked last year at sonywas on crack then..:roll:
Or, he was mistaken, or you misunderstood the answer, or he didn't understand your question.

Some people seem to think that lenses designed for digital cameras will give a different angle of view compared to lenses designed for 35mm cameras. I see this misconception come up often when users are lens shopping for a new DSLR.

That's not the case. It doesn't make any difference if it's designed specifically for digital or not (with a smaller image circle), if you're using both lens types on a DSLR.

Lenses (designed for digital or not) are marked by their actual focal lengths. What changes is the angle of view (apparent magnification), depending on the sensor or film size a lens is being used on.

This "Digital" choice on this chart at Tamron would apply to both lenses designed for digital, or lenses designed for 35mm cameras, if you used them on a DSLR with an APS-C size sensor. Either type would have more apparent magnfication (narrower angle of view) compared to the same focal length lens on a 35mm camera.

http://www.tamroneurope.com/flc.htm

In other words, you'd need to select the "Digital" view to see the correct angle of view for a given focal length lens, regardless of whether or not the lens was designed for a DSLR with an APS-C size sensor. The focal length is the same on both types of lenses on the same camera. Ditto for the angle of view. The only difference is that you're not seeing a so called "cropped view" on a DSLR with a smaller sensor with a digital specific lens. Everything else (including multipliers for comparisons) remains the same.

Only the image circle size changes with a lens designed for digital on a DSLR (so you can't use that lens on a 35mm camera without vignetting). The angle of view (what you get in the resulting image) for a given focal length remains the same with digital or non-digital lenses on a DSLR.

Most manufacturers' lens specs do show angle of view for a lens. But, it represents the angle of view for the camera or sensor size the lens was originally designed for.

If you use a smaller sensor or film size, the angle of view will be narrower (more apparent magnification) for any given focal length.

If you use a larger sensor or film size, the angle of view will be wider (less apparent magnification) for any given focal length.

The only reason to even have a so called crop factor or focal length multiplier is so that users familiar with using lenses on 35mm cameras have a better understanding of how angle of view compares using a DSLR with a sensor smaller than 35mm film.

If 35mm cameras were not so popular, there would be no need to use them at all.

Since you have the same lenses for use on 35mm or smaller sensors with DSLR models from Nikon, Canon, Olympus and KM, giving angle of view is more difficult (since you don't know the camera the lens will be used on with most designs).

Nikon started giving Angle of View for DX lenses assuming an APS-C size sensor would be used (since their DX series are similar to Canon's EF-S series lenses, or KM/Sony's DT lenses, or Tamron's Di II lenses, or Sigma's DT lenses) and they will only work on a camera with a sensor smaller than 35mm film without vignetting).

For example, the Nikon specs for the widest angle of view for the Nikkor 18-70mm f/3.5-4.5G ID-IF AF-S DX lens is shown as 76 degrees at a focal length of 18mm (as you would expect when a lens with a focal length of 18mm is used on a DSLR model with a smaller sensor).

If you look at a non-DX lens, the angle of view shown in the specifications for a given focal length assumes it will be used on a 35mm model.For example, the Nikkor 18-35mm f/3.5-4.5DED-IF AF lens (non DX lens) shows an angle of view in it's specifications of 100 degrees at a focal lenth of 18mm (as you would expect when an 18mm lens is used on a 35mm camera).

The focal length at the widest setting for both lenses is identical. What changes is the angle of view, depending on the size of the sensor/film the lens is being used with.

If you used the non-DX lens (designed for 35mm cameras) on a DSLR, the angle of view would be identical to the DX lens (digital only lens design).

Since we have lenses that can be used on cameras with more than one sensor or film size, it's tougher to give angle of view for these (although they could give multiple angle of views in the specs, showing it for multiple sensor/film sizes).

If 645 format was more popular than 35mm, we may have be seeing "focal length multipliers" to help medium format users make the transition to 35mm, so that users could understand that a lens will have more apparent magnification (narrower angle of view) for any given focal length when used on a 35mm camera versus a medium format model. ;-)


If you really want to know the forumla, here it is:
  • Angle of View = 2 * ArcTan(Film Dimension / (2 * Focal Length * (1 + Magnification)))[/*]
If you want some simple formulas for how larger format films compare to 35mm for a given focal length lens, here some are::

645 focal length x 0.62 = equivalent 35mm focal length
6x6 focal length x 0.55 = equivalent 35mm focal length
6x7 focal length x 0.48 = equivalent 35mm focal length

If you use a lens designed for a given format (as in the new "digital only" lenses designed for a DSLR with a smaller sensor), you don't crop it (as the image circle is designed to match up to the film or sensor). But, you still have to use the exact same formulas for angle of view comparisons for a given focal length lens.

For example, if you use a 50mm lens on a 645 format camera, you'll have a wider angle of view (less apparent magnfication) compared to a 50mm lens on a 35mm camera (it would be like using a 31mm lens on a 35mm camera from an angle of view perpective). You'll have a wider angle of view on 645 film.

Or, to put it another way, you'd have a narrower angle of view (more apparent magnification) using any given focal length lens on a 35mm camera versus a medium format model. You'd need to multiply the focal length of a lens used on a 35mm camera by around 1.7x to compare to the focal length needed on a 645 camera for the same angle of view.

Now what format are we "cropping"? That's one of the biggest misconceptions about digital around.

A lens designed for a 35mm camera will behave exactly the same on DSLR from an angle of view perspective as a digital only lens of the same focal length on that same DSLR from an angle of view perspective (apparent magnfication, what you see on the resulting image).

You'd still need to multiply the actual focal length of the lens by 1.5x if a lens is used on a DSLR with an APS-C size sensor to see what focal length you'd need on a 35mm camera for the same angle of view.

Where the confusion comes in, is because lenses designed for a 35mm camera have a larger image circle compared to lenses designed for a DSLR with an APS-C size sensor.

That doesn't have anything to do with the angle of view you see for a given focal length lens. In the case of a lens originally designed for a 35mm camera, the extra space in the image circle just isn't used with a DSLR using an APS-C sensor.

But, all of this has nothing to do with the Original Poster's issue. That issue is because of the lens design of some of the newer internal focus lenses with a huge range from wide to long in the same lens. The specs for focal lengths assume a focus distance of infinity is being used, since focal length will change with focus distance with this lens design.

If you focus on something closer, it will behave like a shorter focal length lens.

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