Author Topic: a850 lens question  (Read 7016 times)

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Offline jdlaing

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a850 lens question
« on: November 27, 2009, 02:38:43 PM »
I have recently acquired an a850.
I see the DT lenses, when mounted on the 850, automatically revert the capture size in the body firmware, to APS-C size.

My question is:  Is the focal length then 1.5 crop factor?

In other words.......is an 11-18 lens then 22-36 for field of view?

Offline Tom F

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Re: a850 lens question
« Reply #1 on: November 27, 2009, 04:19:01 PM »
Yes

Yes
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Offline JimA

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Re: a850 lens question
« Reply #2 on: November 27, 2009, 04:27:30 PM »
Good job we are friendly round here - I've seen quite violent fights break out over this on other forums

A 20mm lens is a 20 mm lens. The sensor will not change this. It cannot alter the field of view or the focal length; they are part of the physical lens design

We talk of an 20 mm lens on an small sensor as being equivalent to a 30 mm on a 35mm film camera but that does not change the physics

A 20 mm lens of either kind photographing say a building will give the same size image of the building on the sensor.  No matter what kind of lens or whatever kind of sensor

The a850  will i imagine only use the pixels in the centre APS part of the frame if an APS lens is fitted because only the centre of the sensor will be illuminated

But the image of the building will cover the same number of pixels on either sensor with either an APC or fuill frame lens

Offline jdlaing

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Re: a850 lens question
« Reply #3 on: November 27, 2009, 06:44:05 PM »
From what I understand, the in camera correction crops the vignetting when an APS-C lens is used.
Does that necessarily reduce the image by much?

Offline Tom F

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Re: a850 lens question
« Reply #4 on: November 27, 2009, 06:52:22 PM »
It uses the center of the sensor to take a 1.5 ASP-C size image. Much like an image shot with an a700. Actual size the ASP-c is 3984x2656 FF is 6048x4032. With an a700 it's 4272x2848
« Last Edit: November 27, 2009, 07:01:14 PM by Tom F »
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Offline jdlaing

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Re: a850 lens question
« Reply #5 on: November 27, 2009, 09:19:45 PM »
JimA.......

Good job we are friendly round here - I've seen quite violent fights break out over this on other forums

A 20mm lens is a 20 mm lens. The sensor will not change this. It cannot alter the field of view or the focal length; they are part of the physical lens design

We talk of an 20 mm lens on an small sensor as being equivalent to a 30 mm on a 35mm film camera but that does not change the physics

A 20 mm lens of either kind photographing say a building will give the same size image of the building on the sensor.  No matter what kind of lens or whatever kind of sensor

The a850  will i imagine only use the pixels in the centre APS part of the frame if an APS lens is fitted because only the centre of the sensor will be illuminated

But the image of the building will cover the same number of pixels on either sensor with either an APC or fuill frame lens


Okay.

Here are two photos shot at 35mm each.

One is on a 4:3 sensor and the other at full frame on the 850.

http://www.pbase.com/milsuper/sony
« Last Edit: November 27, 2009, 09:23:37 PM by jdlaing »

Offline Reeveyvk

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Re: a850 lens question
« Reply #6 on: June 15, 2010, 09:51:06 AM »
Im confused here, but i have an idea.

When DT lenses (11-18mm) mounted in APC-S sensor, with crop factor (1.5X), the focal length of the lenses became 17-27mm hence narrowed down the angle of view.

When the same lenses mounted in A850, without the crop factor, the focal lenght didnt changed hence the angle of view is wider than when it mount in APC-s sensor. However, the resolution of images cant achieve the max fo the A850.   

The change of the focal length in APC-S sensor was due to the increase of the distance between the sensor and the lenses but not physically on the lences itself.

Offline rogprov

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Re: a850 lens question
« Reply #7 on: June 15, 2010, 10:58:19 AM »
Im confused here, but i have an idea.

When DT lenses (11-18mm) mounted in APC-S sensor, with crop factor (1.5X), the focal length of the lenses became 17-27mm hence narrowed down the angle of view.

When the same lenses mounted in A850, without the crop factor, the focal lenght didnt changed hence the angle of view is wider than when it mount in APC-s sensor. However, the resolution of images cant achieve the max fo the A850.   

The change of the focal length in APC-S sensor was due to the increase of the distance between the sensor and the lenses but not physically on the lences itself.

The focal length of a lens doesn't and can't change, it always is what it says on the lens. What happens when it is fitted to cameras with different size sensors is that they receive a different amount of the image circle projected by the lens. The smaller the sensor the less of that circle is needed to entirely cover it.  This is erroneously called "the crop factor".  But it is a comparison between the image seen by the small sensor and the focal length that would be needed by a lens when fitted to a 35mm camera to give the same field of view.
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Offline Faldrax

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Re: a850 lens question
« Reply #8 on: June 15, 2010, 11:13:37 AM »
The focal length of a lens doesn't and can't change, it always is what it says on the lens. What happens when it is fitted to cameras with different size sensors is that they receive a different amount of the image circle projected by the lens. The smaller the sensor the less of that circle is needed to entirely cover it.  This is erroneously called "the crop factor".  But it is a comparison between the image seen by the small sensor and the focal length that would be needed by a lens when fitted to a 35mm camera to give the same field of view.

Actually, 'The Crop Factor' is one of the better terms for what is happening.
What you get when mounting a 35mm lens on and APS-C camera is a cropped version of the image. This is how APS-C on the A850 / A900 works - it crops the image to only 'use' the portion of the sensor that matches the size of an APS-C sensor.

The confusion arises from the fact that you can also regard this crop as changing the field of view of the lens to be the same as that of a lens with 1.5 x the focal length - and the assumption that the lens somehow also gains the magnification of a lens with 1.5 x the focal length.

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Offline Reeveyvk

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Re: a850 lens question
« Reply #9 on: November 01, 2010, 04:05:49 AM »
For the FF lens, it normally show the angel of view in their data sheet for FF and APS-C sensor. For example, Tamron 28-75 F2.8, the angle of view on FF is 75°-32°, and 54°-21° on APS-C. This mean the coverage of this lens in all focal length on FF sensor is larger than APS-C sensor.

My problem is when using DT 18-70 on A850, i found the coverage of the captured images were not similar as i seen in viewfinder. From the viewfinder, i found the coverage is wide but when i check the image in the LCD, the edges that i seen in viewfinder was been cropped.

So i think DT lens was not suitable for FF sensor eventhough it can be using in FF sensor.

     


Offline Tom F

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Re: a850 lens question
« Reply #10 on: November 01, 2010, 02:42:51 PM »
When you put a DT lens on a A850 or A900 they detect this and automatically go in to APS-C mode and only use the center of the sensor.
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Offline AScot

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Re: a850 lens question
« Reply #11 on: November 07, 2010, 09:33:46 PM »
I figure that the best way to show what happens to a picture taken with a FF lens and a DT lens on an A850/A900 and on a A700 is to try it and see.

The following four pics, #1 to #4 are the as shot untouched results, only reduced in PS5 to comply with the forum guidelines. NO CROPPING HAS BEEN DONE.  The A850 was mounted on a tripod and shot without moving the camera except to change the lenses, see pics #1 and #2 . In pics #3 and #4 the A700 was used and as it is smaller than the A850 the tripod was raised a small amount to keep the pics in the center of the frame. I was very careful to avoid changing the distance from the cameras to the subject. So we can assume that the distance from the sensor to subject is the same in all cases, 24" or (610mm).

The Lenses used were a Minolta 50mm f1.7 full frame lens (FF) and a Sony 16-80 Carl Zeiss (DT) at 50mm. See below the pics for my conclusion.

1/4sec @ 50mm (FF) f4.5, ISO 200. 24.38MP in the photo. A850 + Minolta 50mm f1.7


1/6sec @50mm (DT) f4.5, ISO 200. 10.58MP in the photo. A850 + Sony 1680CZ


1/2sec @ 50mm (FF) f4.5, ISO 200. 12.16MP in the photo. A700 + Minolta 50mm f1.7


1/3sec @50mm (DT) f4.5, ISO 200. 12.16MP in the photo. A700 + Sony 1680CZ


It is interesting to note that the A850 + FF gives a pic (#1) at 50mm (the Epson box in the pic is 3.75" wide on my screen, may be different in yours) and the A850 + DT gives a pic (#2) that is only 1.25x as large (box is 5" in the pic), not the 1.5x crop factor as expected. Note also that the MPixels are less than half the FF, this may be the reason. The MPixel size (10.58MP) agrees with the manual.

The A700 + FF in pic #3 shows a 50mm x 1.5 crop factor (the box is 5.625" wide on my screen and 3.75 x 1.5 = 5.625)) as expected. However in pic #4 we get essentially the same picture as in pic #2, a 50mm x 1.25 crop factor. Both have 12.16 MPixels as expected.

In conclusion I suspect that things do not necessarily work out exactly according to the accepted theories. :D However the above exercise should give the original poster, jdlaing, a better understanding of what to expect with his A850. Note also that the A850 is faster than the A700?
« Last Edit: November 07, 2010, 09:36:24 PM by AScot »
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Offline REX (aka TG)

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Re: a850 lens question
« Reply #12 on: November 12, 2010, 05:19:15 AM »
Sony metion that DT lens can be used on FF but they may give wrong exposure.

The exposure area that the camera measure is much more that it will be capture.

So if you use with a900 a DT lens then is better to change the metering mode of the Camera from  Multi-segment (40 segment Honeycomb pattern) to Center-weighted.

What is said above is not 100% sure since i didn't try this so somebody can have a test and tell us the results
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