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  1. #1
    Junior Member

    Tamron SP AF 70-300mm f/4.0-5.6 Di VC USD got 2 questions

    I got the Tamron SP AF 70-300mm f/4.0-5.6 Di VC USD today and using it on a d7200 body.

    Got 2 questions.
    1. When back button focus is enabled and i push the button it does focus but doesn't keep the focus, it starts focus hunting when i want to push the shutter button. When using the shutter button to focus all works fine. When i use another lens it works fine.

    2. When i look at the exif info i see a focal length of 300 mm when completely zoomed in, shouldn't that be 450 mm when using a FF lens on a APS-C body?


    › See More: Tamron SP AF 70-300mm f/4.0-5.6 Di VC USD got 2 questions



  2. #2
    Senior Member

    Re: Tamron SP AF 70-300mm f/4.0-5.6 Di VC USD got 2 questions

    Set menu A4 to OFF and it will stop re-focusing when you touch the shutter button.

    Second question, No. Its a 300mm no matter what. Every thing is the same as full frame. The lens is 300mm. The distance to the sensor is the same. The sensor is smaller so some of the image misses the sensor. So the captured image is a 'crop'. That crop is then enlarged to a standard size when you view or print. So the 'zoom' is really just a crop and enlargement. Another way to look at it. Take a 35mm negative. make a 4x6 print. Then cut 1/4" off all sides of the negative. Make another 4x6 print from the trimmed negative. The print will look like it was taken with a 450mm because you had to enlarge the smaller negative. A full frame lens is just better glass. A crop sensor lens is made more cheaply. They know some of the image will fall off of the sensor so they design the lens more cheaply. Put that same 300mm crop sensor lens on a full frame camera (and tell the camera to ignore that fact so it doesn't automatically crop the image) and it will work the same as a full frame 300 but the edges will look like crap.
    Thanks/Like TMG1961, BeegRhob Thanks/liked this post
    Best Answers BeegRhob voted best answer for this post
     
    I must have a really good camera.

  3. #3
    Junior Member

    Re: Tamron SP AF 70-300mm f/4.0-5.6 Di VC USD got 2 questions

    Thank you nickt for explaining the second question. Didnt know that.

    Menu A4 is set to OFF and AE-L/AF-L button is set to AF-ON. Also set to AF-C

    But i did test a bit more and i think it is not the auto focus that isn't locked but it is the image stabilization that turns off after i let go of the BBF button. You can hear the stabilization turn on with a soft click and as soon as i release the BBF button you hear another click and then the focus point starts wobbling around. This is worse at longer focal lengths. So i guess the lens image stabilization has a problem with using BBF.

  4. #4
    Staff
    Super Mod
    hark's Avatar

    Re: Tamron SP AF 70-300mm f/4.0-5.6 Di VC USD got 2 questions

    I'm not quite sure I understand everything you mentioned, but the image stabilization will kick in when the back button is pressed and will stop when the back button is released. Image stabilization will also kick in when the shutter button is pressed and will stop when the shutter button is either released or fired. So image stabilization kicks in for both buttons when a body is set for BBF.
    Thanks/Like TMG1961 Thanks/liked this post
     
    Cindy - D750, D500, D7200
    Flickr
    and My 2020 Thread

    Where the Spirit does not work with the hand, there is no art
    -- Leonardo da Vinci



  5. #5
    Junior Member

    Re: Tamron SP AF 70-300mm f/4.0-5.6 Di VC USD got 2 questions

    hark you are correct. Just tested it and works like you said. I always thought that the image stabilization also stayed on when releasing the back button. Now that makes me think about using BBF. Have to give BBF a go and see if it works for me or that just using the shutter button works better for me..


    Thanks all for the explanations. Have a great weekend.

  6. #6
    Senior Member

    Re: Tamron SP AF 70-300mm f/4.0-5.6 Di VC USD got 2 questions

    Quote Originally Posted by TMG1961 View Post
    So i guess the lens image stabilization has a problem with using BBF.
    It shouldn't have a problem. I have that lens. I'll try it later on the d7200 and see if I notice anything weird. I thought VR should stay active as long as the meter stays on, but not sure at the moment. It might vary by lens. I'll try it later.

    edit: just saw your last reply after posting this.
    I must have a really good camera.

  7. #7
    Senior Member

    Re: Tamron SP AF 70-300mm f/4.0-5.6 Di VC USD got 2 questions

    I love bbf by the way. It took me a season of not being fond of it though.
    I must have a really good camera.

  8. #8
    Staff
    Super Mod
    hark's Avatar

    Re: Tamron SP AF 70-300mm f/4.0-5.6 Di VC USD got 2 questions

    Quote Originally Posted by TMG1961 View Post
    hark you are correct. Just tested it and works like you said. I always thought that the image stabilization also stayed on when releasing the back button. Now that makes me think about using BBF. Have to give BBF a go and see if it works for me or that just using the shutter button works better for me..


    Thanks all for the explanations. Have a great weekend.
    Glad the info gave you a little insight. Here is a little something else to consider which sets BBF apart instead of using the shutter button for focusing. Since BBF uses AF-C, as long as the back button is pressed, the lens will continue to focus until you let go of the back button. Once you let go the AF stops. You can then press your shutter button to take the shot. This is helpful if you have a stationary subject and wish to recompose. So BBF is like using both AF-C and AF-S without having to actually switch between the two settings.

    When you have a moving subject, keeping the back button pressed allows the lens to continually focus. If the area on which you are focusing isn't a high contrast area, the lens might have difficulty maintaining focus. And it's possible that is contributing to your lens hunting. That has more to do with a lens not being fast. You mentioned the lens being an f/4.0-5.6. When zoomed out to the tele end of its range, you will be at f/5.6. And depending upon the amount of light in the scene coupled with the contrast on the area you are focusing on, many lenses will struggle to maintain focus - not just that particular lens.

    BUT when you aren't using BBF and are instead using the shutter button for both focusing and firing the shutter, if the body is set to AF-C, you lose the ability to focus and recompose. So there are advantages to BBF.

    When using M, A, or S mode with my D7200, it is in AF-S with the shutter button doing everything. I have U2 programmed to be BBF, Manual Mode with Auto-ISO, and AF-C. So I can quickly switch between being in Aperture Priority with the shutter button to being in BBF with those setting simply by changing the Mode dial to U2.

    At first it was a pain to switch back and forth because I wasn't too fond of BBF, but since wildlife tends to be my main subjects, I finally got used to it. And now I find myself still using BBF for landscapes and other subjects that I never would have considered.
    Thanks/Like nickt, TMG1961, BeegRhob Thanks/liked this post
     
    Cindy - D750, D500, D7200
    Flickr
    and My 2020 Thread

    Where the Spirit does not work with the hand, there is no art
    -- Leonardo da Vinci



  9. #9
    Senior Member

    Re: Tamron SP AF 70-300mm f/4.0-5.6 Di VC USD got 2 questions

    I tried my 70-300. Exactly like yours. I never really paid attention. It should not be a problem. As Cindy described above, you get two 'modes' in one. If you are holding the button down, the issue described does not come into play. If you are using the bb to focus and recompose by letting go of the button, yes, the vr turns off. But you are recomposing and shooting so it should not matter that the vr is off and its harder to hold on target because you are not focusing again. But if that jumpy view is distracting as you recompose, just half press the shutter and vr will kick in again but the focus won't change. I hope that makes sense.
    I must have a really good camera.

  10. #10
    Junior Member

    Re: Tamron SP AF 70-300mm f/4.0-5.6 Di VC USD got 2 questions

    Thanks all for the information. Tonight i tried shooting my first moon photos. Can't say they turned out being any good. Hand shot, 300 mm (no tripod at the moment), was able to lean on something and thought it was ok. But after uploading them to laptop i was disappointed. Will try again next time i got a clear view of the moon. But 300 mm is a bit short for that kind of photos and a tripod is a must.





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