Tripod/Ballhead Questions with 840mm Effective Focal Length

Started 2 months ago | Questions thread
Bob in Baltimore
Bob in Baltimore Senior Member • Posts: 1,068
Re: Tripod/Ballhead Questions with 840mm Effective Focal Length

S. Miller wrote:

Bob in Baltimore wrote:

S. Miller wrote:


  1. if this is the level of sharpness I should expect from the setup I used (X-T3, XF100-400 w/ 1.4x TC - EFL of 840mm, 10-sec shutter delay, Sirui 2204 and BH-40 head)?

I believe that your EFL is 560 mm, not 840 mm with a 1.4 x Barlow & a 400 mm zoom.

Even so, I would expect better detail than you are showing.

My guess is that you are loosing IQ by shooting your zoom lens at its maximum focal length. Most zooms are not at their best when at max or min focal length. Compromises are necessary & making it great at 400 mm would likely have made it pretty awful at 100 mm. Try backing off to 300 mm focal length and see if you don’t like it better. (I shoot my somewhat antiquated Nikon 70-300 at 220 for best results.)

Teleconverters also compromise image quality. The 1.4x Barlow may be contributing to the problem. If you have the energy, try with & without the TC & at 300 mm & 400 mm.

I think you can save a little bit of time by using a shutter delay of just a few seconds. 10 seconds seems unnecessarily long, tho I must say I have never texted longer than 4 seconds, & your system may be more sensitive than mine.

Hi Bob,

I agree that the max end of a 100-400 zoom won’t yield the highest IQ. Ditto for adding a 1.4x TC. I can try to pull back a bit and see if cropping that image further is better than a slightly smaller crop on my current setup.

As to 840mm EFL, this is on the Fuji X-T3, an APS-C camera (1.5x crop).

Ah! Here is the problem. Moving from a full frame to a crop sensor does not change the effective focal length of the lens, even tho people often speak in those terms. When you move from a full frame to a crop camera what changes is the field of view of the lens/camera combination. That is because the crop sensor is smaller than the full frame sensor.

But let's start at the beginning. What does focal length tell you and what can you learn from it.?

The longer the focal length of a lens, the better the "plate scale" is. You can find a nice discussion of that here.

If you have a longer focal length, the image is spread out more in the focal plane. The image is larger. How large is it? That is found by knowing the object's size (1/2 degree in the case of the Moon) and the plate scale of the lens. Plate scale is usually expressed in arc seconds per mm, with the measurements being made at the focal plane. It is a property of the lens.

It is of greater interest to express plate scale in terms of arc seconds per pixel, which is actually a property of the lens and the sensor. It depends on both the focal length of the lens and the pixel density of your sensor - how many pixels across the focal plane. In the case of your XT3 it is 6240pixels across the long (23.5 mm) edge of the sensor. So you need to know the focal length of the lens and the pixel density of the sensor to get the plate scale in arc seconds per pixel.

Here are some calculations from a crop sensor approximating the properties of your XT3 and my telephoto (300 mm), what you actually have (560 mm) and what you quoted (840mm)

Focal Length....... FOV................ Plate Scale.......... Size of the Moon

300 mm ................ 4.4X3.0 Deg..... 2.7 Arc-Sec/Px........ 679 Px

560 mm................. 2.4X1.6 Deg..... 1.4 Arc-Sec/Px..... 1,267 Px

840 mm................. 1.6X1.1 Deg..... 0.9 Arc-Sec/Px..... 1,901PX

In the case of you XT3 and the 560 mm EFL that you actually have, you get about 1.4 arc seconds per pixel. If you had an actual 840 mm lens, you would have about 0.9 arc seconds per pixel. But given typical seeing conditions you are not going to resolve that level of detail and there is little value in oversampling the image, even if you had an actual 840 mm EFL lens.

Returning to the issue of why do people describe their lens focal length in terms of full frame vs a crop sensor ... In this case, if you say your lens has an EFL of 840 mm what you are really saying is that it has the same field of view as you would get with an 840 mm telephoto lens on a full frame camera. That is a useful way to compare lens/camera combinations. It tells you, for example, that you cannot get the entire Pleiades in your field of view with your 840 mm EFL combination.

In summary:

1) Changing to a lens of a longer focal length gives you a better plate scale, regardless of whether you are using a full frame or a crop sensor.

2) Changing from a full frame to a crop sensor changes your field of view. For a lens of a given focal length the FOV gets smaller.

3) It is sometime convenient to use an "equivalent" (not effective) focal length to relate the resulting FOV for a particular lens and sensor combination to what you got in the gold old days with a 35 mm film camera. But, I agree with "Swims" - we would be better off without this confusing use of the term.

Just discovered an excellent article by Nasim Mansurov :

I used the 10 second delay because the remote trigger that I received was the wrong model. Just wanted to ensure I wasn’t causing any camera shake. Normally I would have used 2 sec but wanted to be 100% certain I wasn’t contributing any blur.




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Bob in Baltimore

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