Does image size on the sensor depend on sensor size or 35mm-equivalent focal length?
Asked 2/25/2014
2 views
2 answers
0
If I use the formula
image size on sensor = focal length × subject size ÷ subject distance
then a 50 cm tall subject at 100 m with a 600 mm lens gives an image height of about 3 mm on the sensor.
Is that 3 mm independent of sensor size?
Also, how does this work with compact cameras that advertise a "35mm-equivalent" focal length of 600 mm? For example, if a camera has a 1/2.3" sensor about 4.55 mm tall, would the subject cover about 3/4.55 of the sensor height, or do I need to account for crop factor because the quoted focal length is only an equivalent value?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
7
The answer to your question generally is yes. However as you mention, the ratio of how much of the overall image your subject takes up will depend on your sensor size. What you do have to watch out for is that the "35mm equivalent focal length" should not be used but the actual focal length as quoted on the camera specs - this is usually something in the single digits of millimetres.
The 35mm equivalent is for comparison purposes only because it reflects the equivalent area of 35mm film (or a full-frame digital sensor) - i.e. 36 x 24mm. People tend to be most familiar with the range of focal lengths at this sensor size e.g. 24mm is wide, 50mm is normal, anything about 100mm is telephoto and so on. This is simply the real focal length multiplied by a factor that represents the ratio between the camera's real sensor size and the nominal 35mm sensor size.
Originally by user26360. Source · Licensed CC BY-SA 4.0
user26360
12y ago
0
Generated from our catalog & community — verify before relying on it.
Yes: the projected image size is determined by actual focal length, subject size, and subject distance, not by sensor size.
So if the image formed by the lens is 3 mm tall, it is 3 mm tall regardless of whether the sensor is large or small. What sensor size changes is how much of the frame that 3 mm occupies.
The important catch is this: use the lens’s real focal length in the formula, not the advertised 35mm-equivalent focal length. “35mm equivalent” is only a field-of-view comparison to full-frame.
A small-sensor camera labeled “600 mm equivalent” does not have a true 600 mm lens. Its actual focal length is much shorter, so the image size on the sensor will be based on that shorter focal length.
In other words:
- actual focal length → determines image size on the sensor
- sensor size → determines what fraction of the frame that image fills
- 35mm-equivalent focal length → only tells you the comparable angle of view
So your 3 mm result is correct only if the actual focal length is 600 mm.
Recommended products
UniqueBot
AI12y ago
Your Answer
Related Questions
How do I calculate the focal length needed to frame a subject of a given size at a given distance?
Why does my distance-from-photo calculation give a much smaller result than expected?
Why is aperture usually quoted without a crop-factor equivalent?
Should crop factor be used when calculating camera field of view?
How can I estimate an object's real size from pixel size, focal length, and subject distance?