EF-S lens question

Jim Michael · March 15th, 2015, 06:38 AM

It helps to think about what a lens does. It forms an image of a subject on a plane or flat surface. If the subject is far away the image is formed at the focal length of the lens. So if you have a 50mm lens it forms the image about 50mm from the lens and that image lies on a plane. The image formed is a circle. You can use any portion of the image. If you have a large enough sensor you can use the largest rectangular section of the image that will fit inside the circle. That might be a 24x36mm ("full frame") section. You can also place a smaller sensor inside the circle, in which case you'll use less of the image, resulting in a narrower field of view. For instance, if you are filming a play, the full circle might contain the entire width of a stage so the full frame sensor gets everyone on the stage. A smaller sensor only gets part of the image so you lose some people on the sides of the stage because you aren't using the entire image thrown by the lens.

Now think about the effect of aperture in this scenario. Does changing the size of the sensor in the circle change the amount of light striking the sensor? No, it doesn't. You can substitute any size sensor and it's still the same amount of light falling, you are just using the part of the image where the sensor is located.

Ricky Sharp · March 17th, 2015, 05:00 AM

As others have already (and correctly) pointed out, the aperture value doesn't change. However, my guess of what you may have read was someone comparing crop vs. full-frame and how one would need to achieve a similar depth-of-field.

Basically, if you want to keep the same field-of-view for an image taken with full frame and crop-sensor cameras, the focal lengths will be different. Or, you could use the same focal length, but shoot from different distances (at least I think so). But since DOF is dependent upon focal length and distance, you can change the aperature to help "compensate" for that difference.

But at the end of the day, the resulting images will still look different. e.g. a 135mm focal length on a crop sensor would give you a field of view as if it was a 216mm on full frame. Yet you wouldn't get as much compression of the image as if using a 216mm focal length lens; it's still 135mm.

By all means, please correct anything I've mentioned here; still learning this stuff as well!

Gary Huff · March 17th, 2015, 09:28 AM

Quote:

Originally Posted by Ricky Sharp

As others have already (and correctly) pointed out, the aperture value doesn't change. However, my guess of what you may have read was someone comparing crop vs. full-frame and how one would need to achieve a similar depth-of-field.

I would really like to hear his source for where he got the information, because if this is what he misread, he got it completely backwards too. The "full frame" would "turn into" the f/4.5.to achieve the same DOF as the m43. You have to open up lenses on smaller sensors to get the same DOF as the full frame variety. However, I still think it was the 2x telephoto adapter he was reading about because that's what I got when I googled those aperture relations (2.8 turning into 4.5).

Greg Boston · April 27th, 2015, 04:10 PM

I came across some Youtube videos of a particular photographer pointing out marketing shenanigans of some manufacturers where they point out the 35 mm equivalent focal length, without multiplying the aperture value to match. In the video, listings on Amazon are given as examples.

His contention is that aperture being focal length/diameter of lens, that you break the math if you don't multiply both. Of course, it caused a lot of controversy in the comments of the videos.

In the third video, he responds to critics. Interesting to watch, even if you don't necessarily agree.

I have no affiliation or financial benefit by posting that link.

-gb-

Gary Huff · April 27th, 2015, 04:30 PM

It has to do with matching the depth of field, nothing else.

If you cannot understand this, then you need to go back to the beginning and gain some foundation as to what is going on when you are capturing imagery using lenses and sensors.

March 15th, 2015, 06:38 AM	#16
Jim Michael Major Player Join Date: Dec 2005 Posts: 895	Re: EF-S lens question It helps to think about what a lens does. It forms an image of a subject on a plane or flat surface. If the subject is far away the image is formed at the focal length of the lens. So if you have a 50mm lens it forms the image about 50mm from the lens and that image lies on a plane. The image formed is a circle. You can use any portion of the image. If you have a large enough sensor you can use the largest rectangular section of the image that will fit inside the circle. That might be a 24x36mm ("full frame") section. You can also place a smaller sensor inside the circle, in which case you'll use less of the image, resulting in a narrower field of view. For instance, if you are filming a play, the full circle might contain the entire width of a stage so the full frame sensor gets everyone on the stage. A smaller sensor only gets part of the image so you lose some people on the sides of the stage because you aren't using the entire image thrown by the lens. Now think about the effect of aperture in this scenario. Does changing the size of the sensor in the circle change the amount of light striking the sensor? No, it doesn't. You can substitute any size sensor and it's still the same amount of light falling, you are just using the part of the image where the sensor is located.

March 17th, 2015, 05:00 AM	#17
Ricky Sharp Regular Crew Join Date: Mar 2015 Location: Chicago, IL USA Posts: 180	Re: EF-S lens question As others have already (and correctly) pointed out, the aperture value doesn't change. However, my guess of what you may have read was someone comparing crop vs. full-frame and how one would need to achieve a similar depth-of-field. Basically, if you want to keep the same field-of-view for an image taken with full frame and crop-sensor cameras, the focal lengths will be different. Or, you could use the same focal length, but shoot from different distances (at least I think so). But since DOF is dependent upon focal length and distance, you can change the aperature to help "compensate" for that difference. But at the end of the day, the resulting images will still look different. e.g. a 135mm focal length on a crop sensor would give you a field of view as if it was a 216mm on full frame. Yet you wouldn't get as much compression of the image as if using a 216mm focal length lens; it's still 135mm. By all means, please correct anything I've mentioned here; still learning this stuff as well! Last edited by Ricky Sharp; March 17th, 2015 at 05:43 AM.

April 27th, 2015, 04:10 PM	#19
Greg Boston Wrangler Join Date: Oct 2003 Location: DFW area, TX Posts: 6,117 Images: 1	Re: EF-S lens question I came across some Youtube videos of a particular photographer pointing out marketing shenanigans of some manufacturers where they point out the 35 mm equivalent focal length, without multiplying the aperture value to match. In the video, listings on Amazon are given as examples. His contention is that aperture being focal length/diameter of lens, that you break the math if you don't multiply both. Of course, it caused a lot of controversy in the comments of the videos. Crop Factor: Why you multiply the aperture by the crop factor when comparing lenses - YouTube In the third video, he responds to critics. Interesting to watch, even if you don't necessarily agree. I have no affiliation or financial benefit by posting that link. -gb-

April 27th, 2015, 04:30 PM	#20
Gary Huff Inner Circle Join Date: Jan 2013 Location: Austin, Texas Posts: 2,006	Re: EF-S lens question It has to do with matching the depth of field, nothing else. If you cannot understand this, then you need to go back to the beginning and gain some foundation as to what is going on when you are capturing imagery using lenses and sensors.