This is a very good empirical discussion thread by Alan that helped me understand about reality of TCs with image quality. I have questions indirectly related to what was discussed.
Background:
I am wetting my feet into Astrophotography, specifically Deep Sky Objects (DSO) using DSLR and super-telephoto lenses. While telescopes are the common standard ops for DSO, I want to use Canon's super-telephoto lenses for 3 things mainly, (1) bigger aperture (smaller) F value and better glass for a similar focal length compared to a telescope's F value (caveat we will get below), (2) better weather sealing, (3) sunk investment costs and less amount of connections and devices, thus faster setup. The cons are of course (1) Generally heavier setup as telescopes are generally lighter (caveat below), (2) potentially more costly, but it beats having more stuff by adding more glass in the inventory. However, the costs may offset, if the setup below is successful, since I do not have to purchase multiple telescopes.
With this in mind, my goal is to use 2 lenses: start with an EF 200 F2 that I have and buy an EF 400 F2.8 III, once I get handle on the art. Based on my understanding, in general DSO are shot with focal lengths between 250mm and 1,600mm but can go higher. I am not interested beyond that focal length, because that brings another set of problems. So with the two lenses mentioned above with TCs (I have 2 2X and 1 1.4X EF version III), the setup has capability of potentially up to 2,240 mm focal length, but with significant drop in F stops (F16). This is where very expensive and howitzer heavy telescopes win over the F stops - the caveats above. Thus, limiting myself to 1,600mm, I can see me using 400mm with 2 2x teleconverters while having F11, which is not bad. I might even drop my limit down to 1,120 mm (i.e. 400mm with 1.4X and 2X) with F8, for better quality of pictures. For clarification, I am going to purchase the 400, regardless of getting into Astrophotography.
Camera wise, I either want to use my existing 7D MII or 5D MIV or 5DSR for the setup with permanent full spectrum conversion on the sensor. An R can come later, possibly an R5.
Questions:
Background:
I am wetting my feet into Astrophotography, specifically Deep Sky Objects (DSO) using DSLR and super-telephoto lenses. While telescopes are the common standard ops for DSO, I want to use Canon's super-telephoto lenses for 3 things mainly, (1) bigger aperture (smaller) F value and better glass for a similar focal length compared to a telescope's F value (caveat we will get below), (2) better weather sealing, (3) sunk investment costs and less amount of connections and devices, thus faster setup. The cons are of course (1) Generally heavier setup as telescopes are generally lighter (caveat below), (2) potentially more costly, but it beats having more stuff by adding more glass in the inventory. However, the costs may offset, if the setup below is successful, since I do not have to purchase multiple telescopes.
With this in mind, my goal is to use 2 lenses: start with an EF 200 F2 that I have and buy an EF 400 F2.8 III, once I get handle on the art. Based on my understanding, in general DSO are shot with focal lengths between 250mm and 1,600mm but can go higher. I am not interested beyond that focal length, because that brings another set of problems. So with the two lenses mentioned above with TCs (I have 2 2X and 1 1.4X EF version III), the setup has capability of potentially up to 2,240 mm focal length, but with significant drop in F stops (F16). This is where very expensive and howitzer heavy telescopes win over the F stops - the caveats above. Thus, limiting myself to 1,600mm, I can see me using 400mm with 2 2x teleconverters while having F11, which is not bad. I might even drop my limit down to 1,120 mm (i.e. 400mm with 1.4X and 2X) with F8, for better quality of pictures. For clarification, I am going to purchase the 400, regardless of getting into Astrophotography.
Camera wise, I either want to use my existing 7D MII or 5D MIV or 5DSR for the setup with permanent full spectrum conversion on the sensor. An R can come later, possibly an R5.
Questions:
- What degradation or real benefit of TCs will I see with those 3 cameras? Even if it is a theoretical limit.
- What would be the actual multipliers of the TCs on the cameras, if I use Alan's observations? Again, even if it is theoretical. That would tell me I will not get the max. focal lengths discussed above.
- Will it help to go with a 7D M2 or a 5D M4 or 5DSR, based on the discussion in that thread? The first 2 questions' answers may answer this.
- For let us say the extreme - Max theoretical focal distance of 1600 mm (or 800 mm for my intial setup with EF 200), with need of 2 2X TCs, what noise levels and details should I expect due to decreased apertures?
- Any 2 TC III require a 12 mm extension tube to connect them. With that comes an added problem of losing focus at infinity. How will that play as I stack the TCs? Will I be able to focus at all for DSO? What counters should I take if I cannot?
- Since everything will be manual focus, I do not have to worry about limiting of F stops for losing auto focus on the cameras. But it would be good to know what focus points and at what F stop I will lose such capability. My guess is F8 for all of the 3 cameras, based on quick research.
- Any other challenges you can think of?
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